Step 1: Assign generation levels. Since A is B's brother and B is C's brother, A, B and C all belong to the same generation (call it Level 0), and by extension A is also C's brother.
Step 2: C is D's father, so D belongs to the generation below C, at Level 1.
Step 3: A generation-0 person who is the sibling of a generation-1 person's parent is, by definition, that generation-1 person's uncle (if male).
Step 4: Since A sits at Level 0 (same generation as C, D's father) and is C's brother, A is D's paternal uncle.
Step 5: Checking the remaining options: A cannot be D's father, since that role belongs to C; A cannot be D's brother, since they are a generation apart; and A cannot be D's grandfather, since a grandfather would need to be one generation above C, not level with him.
Correct option: Uncle