The son of M is the father of N and grandfather (Mother’s father) of R. S is the daughter of N and…

The correct choice is: “Great-grandparent (a parent of B’s grandfather)”

M’s son is the father of N. S is N’s daughter and B’s sister, so B is also N’s child. Hence M’s son is B’s grandfather, and M (whose gender is not given) is a parent of B’s grandfather — B’s great-grandfather or great-grandmother.

M is one generation too far to be B’s “Grandfather” or “Grandmother”. “Grandmother’s mother” is wrong because M is the parent of B’s grandfather (a man, M’s son), not of a grandmother.