"Siapa yang mengumpulkan tugas kalian?"

Translation: Who collected your assignments?

[a.jerkov]

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.

[wolfe.mcbr]

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.

[NeoVittori]

exactly. i have talked to natives and they have mentioned that using the plural form of a word isn't always necessary. that sentence can obviously alr mean assingments as in plural given the context of the pronoun "kalian" which is plural.

[NeoVittori]

i typed "who collected your homeworks?" and got it wrong LMAO