Need help with this one.

Must is the correct answer. You want the formal imperative here. Have to is technically acceptable, but it’s informal. This is a formally written document stating a requirement. Should is not imperative. This is a requirement, not a suggestion.

Have to = external obligation (law, regulation, rule etc...). Must = internal obligation (from the person speaking - they strongly believe in the obligation or they are enforcing it). Must would be used if this was an official written notice or regulation, or it was someone creating the obligation. Have to if someone was explaining this obligation as some kind of legal requirement, which would be the case in this example, I guess. Don't worry too much about it, many native speakers are none the wiser.

Hello everyone, I'm new here. I recently took an English exam at my university, but one of my answers was marked wrong. I would like to know why this answer was incorrect and what the correct answer is. Thank you all for your help!

All three answers sound fine to me

"Have to" and "must" both are fine. "Have to" sounds more conversational while "must" sounds formal, like how it might be written into regulations. "Should" could be used as a way to really soften it, making it sound like a suggestion when the sentence is actually stating what must happen. However, the fact that I clarified that with "must" shows the difference. "Should" implies there might be other options and would never be said in regards to something as highly regulated as air travel.

must: something you do because you think is the right thing. have to: youre obligated to do something by an external agent

Shit like this makes me feel bad for you non-native speakers, one of these is slightly more formal than the others, but trust me, we dont care, we will know what you mean, and there are so many ways to get across any given thought in this language itll make your head spin and i think questions like this can be discouraging, because English is apparently a very difficult language to learn. We speak English because its the only language we know. You guys learn English because its the only language we know lol. Obviously from the US here.

Also, it's a run-on sentence.

As others have said, all of them sound fine. Not a great question. That being said, let me make a guess. This kind of construct could be used in two ways: * Passengers "must". If they don't, something bad will happen, and since they'd never want that, they "must" follow. * Passengers "are asked to" do this. If they don't, what will happen is (blah blah). It's not like 'end of the world' serious MUST thing. The options "must" and "have to" fit in each case. "should" sounds like somewhere in between, and I also feel this very slight notion of imposing a moral or spontaneous obligation. IMO the passport thing feels like a "must". Again, this is not a definitive answer or anything. I might be wrong as well.

A and B should both be considered correct. Must is the strongest imperative.

A and B are fine. It's likely most native speakers (at least in America) would use "must".

Grammatically they all work, but I would have chosen "must" as well.

Poor question/answer setup, imo, as both a and b work. c comes close, and most would understand it, but 'should' implies optionality, which is already inferred by giving a consequence for not, meaning a should here would project a scenario where you don't show your passport but can still board, and is technically incorrect.

Did your lesson mention internal obligation vs. external obligation? That would be what's being tested (very poorly) here.

The standard thing that is taught about the difference between must and have to is that must is internal and have to is external. For example, if I say "I must finish this essay today", that could be because I have other work to do tomorrow, even though the deadline is the day after. In that case, "I have to finish this essay by the day after tomorrow" would indicate that the university requires it by then. I assume the correct answer will be given as A for this reason. In this particular instance, I think they are virtually interchangeable in normal usage, so it is just not a great question.

To give you the ‘grammar book’ answer: Use ‘have to’ to express an external obligation - something that is necessary as a rule or law, imposed on everyone as a rule set down by an external authority. Use ‘must’ to express a personal obligation, or when you give / assign someone an obligation.

So, this is super nitpicky but "must" is not the best here simply because there is a caveat after. "Must" implies that there is no caveat, it simply must be done no matter what. "Have to" is the better choice since while you have to do it to get on the plane, you don't have to get on the plane. "Must" would be perfectly fine if the caveat were not present.

Not only is it right, but in situations like this, where it's a formal setting with an official rule, it's very common.

Is nobody going to talk about the awkward "otherwise"? It's the only argument I see for "must" being incorrect. The answer would be "should" because that is the only one that does not impose a hard requirement; passengers have a choice (as implied by "otherwise") when the sentence uses "should."

Native speaker, I would have chosen “must”.

Is it giving instructions to passengers, or is it just describing what happens at an airport? First case I’d say must, second case I’d say have to.

Unrelated, but the clause structure is a bit off here. Otherwise is not a conjunction—it's a conjunctive adverb. This means you can't connect the clauses with a comma; you have to use a semicolon, with a comma after the adverb. **Incorrect**: Passengers must show a valid passport before boarding the plane, otherwise they will not be able to travel. **Correct:** Passengers must show a valid passport before boarding the plane; otherwise, they will not be able to travel.

Comma splice!

Is it “Have to show” and “Must display”? I suppose “Must show” does sound a little funky, but I think it’s unfair to call it incorrect. Any native speaker might choose ‘Must’ here

Must

Modal verbs. Ugh.  Both "must" and "have to" are correct.   However, "must" could be considered more formal (think something written as a rule/policy) and "have to" is informal.  I'm a native speaker, and I'd have chosen "must" as well.  This may help with modal verbs.  https://learnenglishteens.britishcouncil.org/grammar/a1-a2-grammar/have-must-should-obligation-advice

Must is definitely correct answer lol

A and B should both be correct, they mean virutally the same thing. Usually you can pick up on some slight minor detail that changes the meaning, but those two are synonyms in every single sense. For what it's worth, I would have chosen B as well.