There are seven members in a family, four males and four females. A is wife of
C. C has two sons D and
B. Z is the mother of
X. How is X related to C?
I.P is the nephew of
D. R is married to
D.
II. Z is the sister in law of
D.
III. X is the niece of
B. B is the only sibling of
D.

A is wife of C. C has two sons D and B. Statement I. P is the nephew of D. R is married to D. P is son of B. but we do not get information about X. so this statement is not sufficient. Statement II. Z is the sister in law of D. Z is the wife of D. but no info of X, hence this is also not sufficient. Statement III. X is the niece of B. B is the only sibling of D. This statement alone gives us the information that X is the daughter of D and hence granddaughter of C. therefore statement alone III is sufficient to answer the question.