Select the option that expresses the given sentence in passive voice.
Let me buy an expensive bag for my friend.

Correct option is D

The passive voice construction for the sentence Let me buy an expensive bag for my friend is Let an expensive bag be bought by me for my friend. The other options either change the meaning or do not use passive voice correctly.
Active Voice: In Active voice the subject is the one doing the action.
Passive Voice: In Passive voice the subject is the recipient of the action.
If someone gives a command, requests, expresses or desires a wish the sentence is called an Imperative sentence.
To change an imperative sentence into passive voice the structure would be:
V1+ Object. (Active voice)
Let+ subject+ be+ V3. (Passive voice)