Question: Your homework must be turned _____ by the end of tomorrow.
Correct Answer: in
"Turn in" is a phrasal verb meaning to submit or hand over something (e.g., homework, assignments). This is the correct collocation for submitting work.

Incorrect Answers:
- on - "Turn on" means to activate something (e.g., a light or device), unrelated to submitting work.
- off - "Turn off" means to deactivate something, opposite of "turn on."
- up - "Turn up" can mean to appear or increase volume, but not to submit something.