Answer:

Step-by-step explanation:
In passive voice, the subject of the sentence receives the action of the verb.
In this case, the subject is "assignments" and the verb is "done". The word "they" is the agent of the action, which means that it is the person or thing that performed the action.
However, in passive voice, the agent is typically not mentioned.
So,
The passive voice of the sentence "They did their assignments": is
Their assignments were done by them.
Or
Assignments were done by them.