To complete the sentence, we must choose a word that describes the nature of the person's remarks that contrasts with the audience's positive reception to his ideas. Here is the analysis of the options:
Given these definitions, derisive is the correct word, as it introduces a contrasting perspective that fits the context of the sentence.
Fill in the blank with the correct option.
The teacher believed that the student’s sudden lack of interest in class was an ..........., as he had always been enthusiastic and attentive.