There are six children playing football namely A, B, C, D, E and F.
A and E are brothers. F is the sister of E. C is the only son of A's uncle.
B and D are the daughters of the brother of C's father
How is C related to F ?
F is E's and hence A's sister.
So, C is also the son of F's uncle and is, therefore, F's cousin.
So, the answer is (a).