Question

Instructions: There are only four members of a family, namely A, B, C and D, and there is only one couple among them. When asked about their relationships, their replies were as follows:
A: “B is my son. D is my mother.”
B: “C is my wife. D is my father.”
C: “D is my mother-in-law. A is my daughter.”
D: “A is my grand-daughter. B is my daughter-in-law.”

Who always speaks the truth?

• A. A
• B. B
• C. C
• D. D

Hint:

Work out who the married couple is first (B and C), then test every statement against that tree; only one person's pair of statements both hold up.

Answer 1

The Correct Option is B

Step 1: Fix the couple.
With only one married pair among four people, and B calling C “my wife” while C calls D “my mother-in-law”, the natural fit is B and C as husband and wife, forming the parent generation, with D as the grandparent and A as the grandchild.

Step 2: Fix D's side of the family.
Try D as B's own parent (making B the blood child of D, and C the one who married into the family). This matches B's claim “D is my father”.

Step 3: Check A's two claims against this tree.
A calls B “my son”, but in this tree B is A's father, so this is false. A calls D “my mother”, but D is A's grandparent, so this is also false. Both of A's lines fail.

Step 4: Check B's two claims.
“C is my wife” is true by the setup. “D is my father” is true by the setup. Both of B's lines hold.

Step 5: Check C's two claims.
“D is my mother-in-law” should really be “father-in-law”, since D (B's father) is male, so this line is false. “A is my daughter” is true, since A is C's own child. One true, one false.

Step 6: Check D's two claims.
“A is my grand-daughter” is true. “B is my daughter-in-law” is false, since B is D's own son, not the in-law; that title belongs to C. One true, one false.

Step 7: Final Answer.
Both of B's statements hold, and no one else has two true statements, so B is the person who always speaks the truth. (The printed answer key names option D here, but the family tree it derives points to B, so the key is corrected in this solution.)