C is the daughter-in-law of A who is the grandmother of F means C is the mother of F. But F is the son of B. So, B is C's husband. But C, the lawyer, is married to the Doctor. So, B is the Doctor. F, the Accountant, will be the son of B and C. E is the unmarried Engineer. So, the other married couple can be that of grandmother of F i.e., A and D. But D, the Salesman, is married to the Lady Teacher. So, D, the Salesman, is the grandfather of F, father of B and the husband of A, the Lady Teacher.

D is the grandfather of F.