A is the son of B. B sister has a son D and a daughter E. F is the maternal uncle of D. How is A related to D?

Introduction / Context:
This blood relation problem asks how A is related to D in a small family involving a parent, a sibling and their children. It is a standard example of how children of siblings are related to each other, which is a key idea in many reasoning questions.

Given Data / Assumptions:

• A is the son of B, so B is a parent and A is a male child.
• B sister has a son D and a daughter E. So D and E are children of B sister.
• F is the maternal uncle of D, which confirms that F is a brother of D mother, but this extra fact is not essential to relate A and D.
• We assume that B and B sister share the same parents and are in the same generation.

Concept / Approach:
The children of two siblings are called cousins to each other. Here, B and B sister are siblings. A is the child of B. D is the child of B sister. Therefore, A and D are children of siblings and thus cousins. The problem also states that A is male, so he is a cousin brother, but among the options the generic term cousin is used.

Step-by-Step Solution:
Step 1: Identify the sibling pair: B and B sister are brother and sister in the same generation.Step 2: A is the son of B, so A is one generation below B.Step 3: D is the son of B sister, so D is also one generation below B and the sister.Step 4: Since A and D parents are siblings, A and D are cousins.Step 5: The required relation from A side to D is therefore cousin.

Verification / Alternative check:
Place B and B sister on one line as siblings. Draw a downward arrow from B to A and another from B sister to D. Both arrows go to the same lower generation. There is no direct parent child or sibling relation between A and D, but their parents being siblings means A and D are first cousins. This is the most direct and widely accepted definition.

Why Other Options Are Wrong:
A is not D uncle, because an uncle would be a brother of one of D parents, that is, be in the parent generation, not in the same child generation. A is not D nephew for the same reason, that would require A to be one generation lower than D. A is not D brother, because they do not share the same parents; their parents are siblings, not the same person. Son is completely incorrect because there is no parent child relation from D to A.

Common Pitfalls:
Some candidates rush and confuse "sister of B" with "mother of A" or similar, which leads to wrong generational links. Always separate clearly: B and B sister are siblings; their children are cousins. The mention of F as maternal uncle of D is an extra detail that can distract you if you focus on it too much. For the relation between A and D, it is enough to know who their parents are.

Final Answer:
A is related to D as a cousin.