The correct option is A Who
The correct answer is Option A.
WHO is used when we ask about a person.
In the given sentence, the speaker wishes to know the person with whom Andy's little brother plays.
Hence WHO - Option A is the correct answer.
The remaining options are incorrect because:
Option B: WHY is used to ask about a reason.
Option C: HOW is used to ask about manner of something.
Option D: WHERE is used to ask about place.