The boxer was . . . . . . . . his guard when his opponent threw a right book. A. on B. off C. of D. at

The correct answer is: B. off

The boxer was off his guard when his opponent threw a right hook. This means that he was not prepared for the attack and was therefore more likely to be hit.

The other options are incorrect because they do not make sense in the context of the sentence.

• On his guard means that the boxer was prepared for the attack.
• Of his guard is not a grammatically correct phrase.
• At his guard means that the boxer was aiming for his opponent’s guard.

I hope this helps! Let me know if you have any other questions.