The correct answer is C. explain.
To expostulate means to argue or protest strongly. In the sentence, “Before I could expostulate he had spoken again,” the speaker is trying to say that they were about to argue or protest something, but the other person spoke before they could.
The other options are not correct because they do not fit the context of the sentence. To postulate means to assume something to be true without proof. To protest means to express disapproval or objection. To disclose means to make something known or reveal a secret.
In the sentence, “Before I could expostulate he had spoken again,” the speaker is not assuming something to be true without proof, they are not expressing disapproval or objection, and they are not revealing a secret. They
are trying to argue or protest something, but the other person spoke before they could.