How is this true

Dear student,

In option (A) M is the grandfather of R as S is the father of R and S is the son of M.
In option (B) M is the brother of R.
In option (D) M is the father of R but R is the mother of S. Hence R is the daughter of M.
In option (C) M is the mother of R and R is the father of S. Hence R is the son of M.

Hence option (C)

Regards