There are two generations in the family. There are 6 members: 3 males ($T$, $S$, and one other) and 3 females ($U$, $Q$, and one more). There are two couples, and the remaining two are unmarried siblings. The following individuals are definitely married: $P$, $Q$, and $R$. $T$ is definitely single. We do not know the marital status of $S$ and $U$. From the given information:

$U$ is $P$’s daughter, which means $P$ belongs to the senior generation and $U$ belongs to the junior generation. $Q$ is $R$’s mother-in-law, meaning $Q$ and $P$ are part of the senior generation, and $R$ is in the junior generation. This helps us deduce the following family structure:

$U$ and $T$ are unmarried siblings. $R$ is $S$’s wife. Hence, the family arrangement is clarified, and option (b) is the correct answer