The sentence presents to us the contrast between a normally _______ speaker who became rather _______ when he was confronted with the intense stares of his audience. The best choice of words is given in option B, which says that the speaker, who is normally outspoken, became reticent (or quiet) when he was confronted with the intense stares of the audience. The correct answer is option B. All other options fail to suit the grammar or the context in the sentence.