If A+B means A is the brother of B;
A-B means A is the sister of B;
A×B means A is the father of B.
Which of the following means that ‘C is the son of M’?

Option I shows : N is the father of C and M is the sister of N. Hence this has to be wrong.

Option II : C is the brother of N & N is the father of M . Hence this has to be wrong.

Option III : M is the sister of F & F is the father of C . hence this also wrong.

Option IV : M is the father of N, N is the sister of C & C is the brother of F, this implies C is a male and N is female such that {M,N} are kids of M (father)

Therefore answer is option IV.