It's specifically asking for the plural though, which means that たち is needed.
This is an English-as-a-First-Language perspective. たち is used is certain patterned ways, but in my experience most plural nouns are naked. You just know from context if its plural or even matters.
It's not specifically asking for plural; it's just plural. Without context, it is impossible to say whether 大学生 or 大学生たち should be used, so it should accept both.
Specifically, while in English, the plural must always be used when there are multiple students, in Japanese the singular would typically be used unless it matters and/or cannot be inferred that there is more than one student. So in a "Look at those students" type of context, the English word "students" would translate to simply 大学生.
Do you mean both are marked as correct in Duolingo or both are correct in the absolute sense? If the former is what you mean, I tried 大学生 and is still not accepted.
While "大学生たち" can be used, I never heard it in the four years I lived in Japan. In fact, I rarely heard "たち" used, which surprised me because classes in the US act like it is common. Duolingo should drop this question altogether because it sets up an unrealistic expectation
Not my class in the US. I studied Japanese for five years in Hawai'i, and we never used たち except for pronouns. Perhaps the proximity improves the quality of the class.
No need to represent plurality. This is incorrect. Do Japanese native speakers prepare this content?