M introduces R as the son of the only brother of her mother's husband. How is R related to M?

Correct — A.
Rule: start at M and resolve one link at a time.

M's mother's husband = M's father.
The only brother of M's father = M's paternal uncle.
That uncle's son = M's cousin → option (a).

Why the others are wrong

• (b)
  Brother — Brother would need R to be the son of M's father. R's father is the father's brother, a different man, so R and M share grandparents, not parents.
• (c)
  Uncle — Uncle is R's father, the brother of M's father. R is one generation below him, on M's own level.
• (d)
  Nephew — Nephew is the son of M's own brother or sister. R's father is M's uncle, not M's sibling.

"Her mother's husband" is a roundabout way of saying father. Decode it first and the chain shortens to father → father's brother → his son.

The child of a parent's brother or sister is a cousin, on the same generation as you.

Key facts

• Mother's husband is read as father.
• Father's brother = paternal uncle.
• A child of your parent's sibling is your cousin, on your own generation.

Common traps

• Stopping at the uncle and choosing Uncle, which is R's father
• Reading 'son of the only brother' as the brother himself

No directly related past PYQ was found.