If R is the mother of girl P and the sister of A, and A is the only son of D and the father of girl C, then how is P related to A?

Correct option is D

Given: 
From the given information following family diagram will be - 

To explain why A is P's uncle, let's analyze the relationships step by step:

R (Mother of P) is A's Sister:

It is stated that R is the mother of P and the sister of A.
This makes A the brother of R.
The Brother of One's Mother is an Uncle:

By definition, the brother of someone's parent is their uncle.
Since A is the brother of P's mother (R), A is P's uncle.