A is the only daughter of B, who is the only son of C. How is A related to C?

Correct — C.
Rule: count generations down from C, then apply A's gender.

B is the only son of C: one generation below C.
A is the only daughter of B: two generations below C.
A is C's son's daughter, so A is C's granddaughter.

Granddaughter → option (c).

Why the others are wrong

• (a)
  Daughter — Daughter is A's relation to B, not to C. C sits one generation higher, so B's daughter is C's granddaughter.
• (b)
  Sister — Sister would put A in C's own generation, sharing C's parents. A is two generations below C.
• (d)
  Cousin — Cousin would need B and one of C's parents to be siblings. Instead C is B's parent, two generations above A.

Blood-relation chains are solved by drawing a small family tree. Put C at the top, B one level down as C's son, and A one level below B as B's daughter.

Two levels down means a grandchild, and A's gender makes it granddaughter. C's own gender is never given and does not matter: A is the granddaughter of a grandfather or a grandmother alike.

Key facts

• A son's daughter is a granddaughter.
• Two generations below a person is that person's grandchild, whatever that person's gender.
• Only son and only daughter rule out brothers and sisters but do not change the chain.

Common traps

• Answering daughter by stopping at B instead of going up to C
• Waiting for C's gender, which the question never gives and does not need

No directly related past PYQ was found.