Can the answer be "agreed"?

For me it would sound most natural to say something like “…to which the students replied in the affirmative…”. If someone asks a question, you can’t “agree” with them. “did you understand?” “I agree” 🫤

Something like “said yes” would be better. There’s nothing to agree with here, it’s a yes or no question. If the teacher said “it seems you understood the assignment” then they might agree. The question itself is poorly done, too. It should be “by completing **the** sentence” and it’s not dialogue “between a teacher and a student” but a teacher and multiple student**s**. Also, there shouldn’t be a comma before “by.”

I would use the word 'affirmed' for that sentence

"Approved" "Acknowledged"

Honestly the one that works for me is “affirmed” or maybe “confirmed” if it was just a one word answer. “Understood” may also be it but it’s repetitive.

“The teacher asked … to which they _____.” is already awkward. What does “which” refer to here? Maybe “to which they responded that they had.”

Agreed would work, but the sentence as a whole makes no sense. Especially since the only way students would express gratitude about homework is if there was no homework.

I think ‘agreed’. Could be ‘assented’, ‘acceded’, possibly ‘consented’. But in fact they are all so formal that the sentence is very rare and archaic in British English.

I would probably say "confirmed" there. Teacher: Did you understand it? Students: Yes, we understand. Thank you for making sure.

It could grammatically be “agreed” but it wouldn’t really fit that well. Honestly, not a lot of words would fit very well here, and the question is worded strangely in the first place which leads me to believe the question wasn’t written by a native speaker and the answer they are probably looking for is likely not anything a native speaker would say. If I had to pick a word I’d say “rejoiced”. Even though it’s still not a great answer. “Answered” doesn’t really work because the last clause of the sentence implies that the missing word has something to do with everybody being happy or relieved. “Rejoiced” doesn’t work that well either though because from context the word has to contain some sort of response to a yes or no question (while also expressing the excitement of the group). But it could be argued that if everyone rejoiced then everyone agreed that they understood, but that would really only work if the question wasn’t really a serious question and the teacher was just trying to score some free points by replacing difficult assignment with an easier one.

Yes, it could be “agreed.” The student are expressing their gratitude for having an assignment that they can understand. By saying, “We all got it” to the teacher, they are in agreement. Alternatively, you could say “concurred,” but that is essentially a synonym with “agreed.”