Why the answer is E? couldn't it be A?

Also, "had investigated" is past perfect and talks about an event that finished in the past before another event. "Have investigated" (present perfect) is better here as the studies were conducted in the past, but present perfect emphasises their effect on the present (ie that parenting behaviour can be transmitted intergenerationally). Compare: Past perfect: "I had studied teaching for a year before I got the job at the university." (two past events) Present perfect: "I have studied teaching for a year, so I feel pretty confident." (past event's effect on the present)

It depends on whether this is a class on science or grammar

Is this a YDT question?

No, the answer can't be A. There's a difference between "*must* be transmitted" and "*could* be transmitted." Must is an imperative - it will happen, no matter what. That doesn't make sense in this context.

As an aside, a native speaker would generally word this as "Why is the answer E?"

Tentatively ignore people talking about tenses being the reason for A not being correct, they are well meaning but wrong. A is incorrect because of the use of 'must' as a definitive. Scientific studies generally do not provide certainties, even if the chance of something happening is 99.9999999% possible. A reputable study will only ever use terms such as could, may, suggests, or shows when relating a cause to an effect. In fact, every incorrect answer is wrong because of the use of definitives when assigning cause and effect. The only one incorrect through both terms is B but even for this, 'investigate' is only wrong because of a tense mismatch (please note that while tenses do not always have to match, in this particular case they should do so due to the use of 'found' instead of 'find').

"had" is usually awkward when there is no other past event to compare to. this is a little tricky here, because you may think that "found" is another past event you could compare to, but grammatically "found" is sharing the helping verb. "had investigated... and found" means "had investigated ... and had found". so you have two perfects, and you're still looking for another past event. it's not there.

Also OP, re; "That's how I feel like" you have two options: A) How it feels - How does it feel? B) What it feels like - What does it feel like? This mistake is extremely common because in other languages the equivalent question word for /\_\_\_\_\_\_ it feels (like)/ is always "How". \-Polish for example; Jak to wygląda Się where /Jak/ = How. ​ But in English we are using "How" for Adjectives, "What" for things, ideas, people etc and "like" to make comparisons. \-How do you feel? "I feel sore" \-What do you feel like? "I feel like I just ran a marathon". In the first you simply describe your feeling. In the second you compare your feeling to one you'd experience after a marathon (but you didn't). "I'm hungry" "I'm hungry like the wolf" (song lyrics), you're comparing yourself to a wolf. This may be contrary to what you were taught in language school, but in my experience non-native teachers are making the same mistake and passing it on.

Studies only tell you about possibility, rather than about assurance. Their conclusions never reach 100% certainly because they always have limited data and approaches, which aren’t enough to encompass the whole reality. Therefore, I think ‘must be’ is an inappropriate phrase in that context.

It can't be A, 'cause A says those practices MUST be transmitted, which is wildly incorrect; can't be B or C, because they're not in past tense; and can't be D, because "would have been" doesn't make sense in that context

If a study uses a language that implies certainty beyond doubt it would be rejected or regarded negatively. You will never see one reputable research say “A *must* be the result of B” but rather that “A could/is likely to be the result of B”. This isn’t only a grammar question but a comprehension question too (as in, do you understand the implication of each wording rather than which would make the sentence grammatically sound) — assuming you are a college student.

Is this question just totally on its own with no related reading?

The past perfect is used when one past event has relevance to another reference point in the past. This means it will almost always be paired with another clause or sentence in the simple past which established that reference point. "Had investigated" here would leave us asking "When?"

A is valid. It's a bad question.

Had investigated doesn’t sound right as studies is a plural noun