“Who’s the conductor of this bus?” he said, and paused for a reply.

The sentence “Who’s the conductor of this bus?” is a question. The speaker is asking for information about the identity of the conductor of the bus. The speaker pauses for a reply because they are waiting for someone to answer their question.

Option A is incorrect because the speaker is not stating who the conductor of the bus is, they are asking who the conductor is.

Option C is incorrect because the speaker is not asking whether or not someone is the conductor of the bus, they are asking who the conductor is.

Option D is incorrect because the speaker is not asking who the conductor of that bus was, they are asking who the conductor is.