The correct option is D - Q is a sister of the husband of S.

[as per provisional answerkey]

Step-by-step Trace

To solve this blood relation problem, we map the connections step-by-step:
1. P's Children: The question states P has a son and a daughter. It also states Q and R are children of P. Therefore, one of {Q, R} is male and the other is female.
2. S and R's Relationship: S is R's spouse. This means S and R are a married couple.
3. S and T's Relationship: S is the mother of T. Since S is a mother, S is female. Consequently, her spouse R must be male (the son of P).
4. Identifying Q: Since R is the son of P, and P has only one son and one daughter, Q must be the daughter of P.
5. Connecting Q to S: R is the husband of S. Q is the sister of R. Therefore, Q is the sister of the husband of S (sister-in-law).

Why the other options are incorrect

Option (a) - Q is a sister of S: This is incorrect because Q and S belong to different families by birth; Q is P's daughter, while S entered the family by marrying P's son, R.
Option (b) - Q is a daughter of S: This is incorrect because Q and S are in the same generational level. Q is the sister of S's husband, not her offspring.
Option (c) - Q is the mother of S: This is incorrect because Q is the daughter of P, making her the same generation as S's husband. S's mother is not mentioned, but Q cannot be her mother.

Key Concept

Family Tree Mapping: Identifying gender and generational levels through marital and sibling links to determine specific in-law relationships.