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.