Why not C but E

If you wanted to make this sentence use the word "unless", you would have to rephrase the entire second half of it. "unless they eat it to dangerous excess" would be correct. The way things are phrased in the question is the opposite though because "they do not", so "unless" is nonsense - you implied it has health benefits ONLY if they eat it in dangerous amounts.

Because of the do not. Remove the do not and it would be C not E.

It’s E because it gives a term or a condition or additional info: “Unless” is used for contradiction

Unless cannot be used with a negative form.  “Dark chocolate has certain health benefits for those who eat it, unless they eat it to a dangerous excess” would have worked. 

Unless doesn't go with a negative clause. You won't come in unless you have a ticket. Not 'You won't come in unless you don't have a ticket'

It should have been lest not unless

If 'unless' is put in the gap, then the sentence can be rephrased as: "Dark chocolate has certain health benefits for those who eat it to a dengereous excess." This is, I would say, a textbook example of two negatives ("unless" and "not") cancelling out and becoming a positive.

“Unless” would be grammatically correct but it just happens to make the sentence contradictory. For example “my 4 year old child is 5 years old” is a grammatically correct sentence. But “my 4 year old child is not 5 years old” is grammatically correct too, and makes sense.

Provided that means if, so it is the safe choice.

Meaning-wise it is correct, but the second half would need to be reworded.

“Dark chocolate has certain health benefits for those who eat it, unless they do not eat it to a dangerous excess” means “dark chocolate has certain health benefits for those who eat it, so long as it is eaten in dangerous excess”. It’s the opposite of what you’re trying to say. The use of “unless” means “mitigate the first clause if the second clause does not occur” while the use of “provided that” means “mitigate the first clause only if the second clause does occur”.

Grammatically C works, but it changes the meaning into something inaccurate and contradictory. It’s a double negative, and English usually treats those literally. You’d be saying that dark chocolate only has health benefits to people who eat it to a dangerous excess. (If it gave you benefits *unless* you do *not* eat it to dangerous it excess, then to get the benefits you *would* need to eat it to a dangerous excess.) Which makes no sense and is not the intended point of the sentence. Parsing out that negativity is confusing, so we tend to avoid such constructions. We might say “I won’t kiss you unless you shave,” but it’s confusing to say “I won’t kiss you unless you do not forget to shave.” Uh… what?

"Provided that" is a positive conditional and unless is a negative one. Like in programming/logic, "if (foo)" versus "if (!foo)". More examples: You will succeed provided that you work hard. You will not succeed unless you work hard.

Others have provided the answer, but I want to add that this is unlikely to be a useful addition to the sentence. Obviously, anything consumed to a dangerous excess would be unhealthy. A better example would be Conjunctions are a useful language mechanism, provided that the author uses it to add informative value to the sentence's main idea.

Provide = confunction "if" Provided that (X)

Simply speaking: In case they do NOT eat… Unless they eat (without not)

C makes no sense here. “Unless” implies that there is an action that can be taken to counteract the benefits of eating dark chocolate. Not eating dark chocolate isn’t an action, but a lack of an action. I believe it is also a double negative. “Provided that” means that if one follows instructions, the dark chocolate will bestow benefits on the eater

“Unless” would mean that you’d need to eat it in dangerous excess to actually get the health benefits. “Provided that” basically assumes the second sentence is true (aka on the pretext/assumption that you aren’t eating it too much)

As a native english speaker, e is correct, c would mean they have to eat it in excess for the health benefits.

Here's why: Explanation: C (unless) means "except if," which introduces a condition where something would not happen. If we use "unless" here, the sentence would imply that eating dark chocolate to excess is the only condition that negates its health benefits, which doesn't make logical sense. It would mean the benefits still apply even if consumed dangerously, which contradicts the intended meaning. E (provided that) means "on the condition that," which fits the intended meaning of the sentence: dark chocolate has health benefits, but only if it is not eaten to dangerous excess. This expresses the conditional requirement clearly and aligns with the context.

This made me laugh

My first thought was "as long as" sounds like "unless" lol

Unless doesn't go with not It's kind of like saying "as long as it's not true that..." followed by a "they don't" making a double negative. Not all double negatives are wrong, but this one sounds bad.

As written, "unless" expresses exceptional circumstances that contradict the prior point, and so here generates a double negative because of the "not" in the second clause. A similar meaning to "useless" is given by "except if." Consider this sentence: >Dark chocolate has certain health benefits for those who eat it, ***except if*** they do ***not*** eat it to a dangerous excess. The "not" is what causes the problem here. There is a double negative that makes the sentence's meaning incorrect, suggesting there's a problem if people do *not* eat it to a dangerous excess. An acceptable use of "unless" would be: >Dark chocolate has certain health benefits for those who eat it, unless they eat too much of it.

Get rid of the "do not" and unless works.

"Unless" means "except if" - the sentence "...except if they do not eat it to a dangerous excess" sounds like a double negative - like you're saying you *should* eat to a dangerous excess. You could fix this sentence by removing the "do not" to make that work, but that isn't the question that was asked. "Provided that" is a phrase that means something will happen only if another thing happens - it agrees with the statement "do not eat..." to make it clear dark chocolate only has health benefits when eaten in small amounts.

The “do not”. If you took away the “do not”, then C would be correct.

Think carefully about the meaning. The sentence with "unless" would mean "Dark chocolate has certain health benefits _if it is_ eaten to a dangerous excess."

C reads incorrectly to me, so I'll let someone else answer. As a native speaker, though, I agree with your E.