The answer is wrong. It should be: "Who collected your assignment." -> singular, not plural.
It could have been that the speaker is asking one group of people about a group assignment.
For the answer to be explicitly plural, the Indonesian sentence should have been: "Siapa yang mengumpulkan tugas-tugas kalian." Otherwise, it's not clear whether the speaker is asking about a single assignment or a collection of assignments.
Editor, please review this.
I partly agree. Without context it could be singular or multiple assignments, so both should be accepted here.
In real life however i feel there would never be a context where its ambiguous. if there is no group assignment then we can interpret tugas as plural without needing to duplicate.