$M$ and $N$ had four children $P$, $Q$, $R$ and $S$.
It means $M$ and $N$ are couple.
Only $P$ and $R$ were married.
They had children $X$ and $Y$ respectively.
It means child of $P$ is $X$ and child of $R$ is $Y.$
If $Y$ is a legitimate child of $W$ we can conclude that $W$ and $R$ are couple.
- $M$ is the grandmother of $Y-$ Possible, can also be grand father.
- $R$ is the father of $Y-$ Possible, can also be mother.
- $W$ is the wife of $R-$ Possible, can also be husband.
- $W$ is the wife of $P{\color{Red}{-\text{False}}},$ $W$ is either wife of $R$ or husband of $R.$
So, the correct answer is $(D).$