Pointing to a boy, a man said, “He is the son of my wife’s sister’s husband.” How is the boy related to the man?

Correct — B.
Rule: a wife's sister's husband is a brother-in-law, and a brother-in-law's son is a nephew.

My wife's sister → my sister-in-law
Her husband → my brother-in-law
His son → one generation below me: my nephew → option (b)

Why the others are wrong

• (a)
  Son — Son would make the speaker the boy's father. The father named is the wife's sister's husband, a different man.
• (c)
  Brother-in-law — Brother-in-law is the boy's father's relation to the speaker. The boy stands one generation lower, as a nephew.
• (d)
  Cousin — Cousin would put the boy in the speaker's own generation. He is the son of a man in that generation, so he sits one generation down.

Blood-relation statements are unpacked one possessive at a time, starting from the speaker: wife → her sister → her husband → his son.

Track generations as you go. The wife, her sister and the sister's husband sit in the speaker's generation; the son sits one below.

The son of a brother-in-law or sister-in-law is a nephew, the same word used for a brother's or sister's son.

Key facts

• Nephew: a son of one's brother or sister, or of one's brother-in-law or sister-in-law.
• A wife's sister's husband is a brother-in-law, in the speaker's own generation.
• Cousins, brothers-in-law and sisters-in-law share your generation. Nephews and nieces sit one below.

Common traps

• Stopping at 'husband' and answering Brother-in-law, which is the father's relation to the speaker, not the boy's.
• Answering Cousin: that is what the boy would be to the speaker's children, not to the speaker.

No directly related past PYQ was found.