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.
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