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

Correct — C.
Rule: move one generation per link, and give a gender only where the question states one.

B is the daughter of A, so A is B's parent.
A is the son of C, so C is A's parent.

C is two generations above B, which makes C B's grandparent. C's gender is never given, so the neutral word is what the facts support → option (c).

Why the others are wrong

• (a)
  Mother — Mother is one generation above B. C is two generations above: C is the parent of A, and A is B's parent.
• (b)
  Father — Father is one generation above B, and that place belongs to A, who is male because he is called a son. C sits a generation higher.
• (d)
  Uncle — Uncle is a parent's brother, in A's own generation. C is A's parent, not A's sibling, so C cannot be B's uncle.

Blood-relation chains are read one link at a time. Each 'is the son of' or 'is the daughter of' link moves one generation up, from child to parent.

Here the links run B → A → C, so C is two generations above B.

The words also carry gender: A is a son, so male, and B is a daughter, so female. Nothing is said about C, which is why the key uses the neutral grandparent rather than grandfather or grandmother.

Key facts

• A parent's parent is a grandparent, two generations up.
• Son and daughter fix the gender of the child in a link, not of the parent.
• An uncle is a parent's brother, in the parent's generation.

Common traps

• Stopping one generation short and answering Father
• Assuming C is male because A is described as a son

No directly related past PYQ was found.