Who is the son of A? Statements: I. B is A's brother. II. C is the daughter of…

Concept:

In Data Sufficiency questions, a statement (or combination of statements) is 'sufficient' only when it lets you arrive at exactly one definite answer to the question asked. If, after using the given information, the needed fact is never actually stated, that statement or combination is 'not sufficient.'

Application:

1. Statement I alone: B is A's brother. This only establishes a sibling relationship between A and B; it gives no information about any child of A, so A's son cannot be identified from Statement I alone.

2. Statement II alone: C is the daughter of B. This only tells us about B's family; without knowing how B relates to A (not given in Statement II), it says nothing about A's son either.

3. Both statements together: Combining them tells us B is A's brother and C is B's daughter — meaning C is A's niece, not A's son. Even together, the two statements describe A's sibling and A's niece, never A's own son.

Cross-check:

Since none of the three combinations (Statement I alone, Statement II alone, both together) ever states any child of A, none can determine who A's son is — confirming that the two statements, even combined, remain insufficient to answer the question.