Blood relations – "sister’s son" chain: D said, "A's father is the only brother of my sister's son." How is A's father related to D?

Introduction / Context:
We map "my sister's son" to "my nephew," and then take the only brother of that nephew, and compare to D.

Given Data / Assumptions:

• D has a sister; her son is D's nephew.
• A's father is the only brother of that nephew.

Concept / Approach:
Brothers of one nephew are also nephews (to D). Hence A's father is D's nephew. Then A is D's grand-nephew/niece; but the question is about A's father and D.

Step-by-Step Solution:

Verification / Alternative check:
A simple family graph confirms that both siblings (the nephew and his brother) are nephews to D.

Why Other Options Are Wrong:
Cousin/uncle invert or move to wrong generation; data is fully adequate.

Common Pitfalls:
Reading "sister's son" as cousin; it is a nephew from D's perspective.

Final Answer:
Nephew