The thief, . . . . . . . . they caught, was sent to the prison. A. who B. which C. whose D. whom

The correct answer is: A. who

The sentence is about a thief who was caught and sent to prison. The pronoun “who” is used to refer to people. In this case, it is used to refer to the thief.

The other options are incorrect because they are not used to refer to people.

• Which is used to refer to things. In this case, it would not make sense to say “The thief, which they caught, was sent to the prison.”
• Whose is used to refer to possession. In this case, it would not make sense to say “The thief, whose they caught, was sent to the prison.”
• Whom is the objective form of “who.” It is used to refer to people as the object of a verb or preposition. In this case, it would not make sense to say “The thief, whom they caught, was sent to the prison.”