Step 1: Recall the established roles.
From the family tree, B and C are husband and wife, with B's own statement “C is my wife” fixing B as male and C as female.
Step 2: Understand what the question is asking.
“How is B related to C” asks for B's role as seen from C's side, which is the mirror image of “C is my wife”.
Step 3: Reverse the relationship.
If C is B's wife, then by definition B is C's husband. The words “wife” and “husband” describe the two ends of the same marriage relationship, not the same end twice.
Step 4: Eliminate the other options.
“Father” and “mother” would only fit a parent-child pair, not a married pair, so both are wrong. “Wife” describes C's role toward B, not B's role toward C, so it is also wrong here.
Step 5: Final Answer.
B is C's husband, so option D is correct. (The printed key lists option C, wife, but that is B's own description of C, not the reverse relation the question asks for, so the key is corrected here to option D.)