When speaking, the instructor should use voice fluctuation to:

Using voice fluctuation is an effective technique for an instructor to emphasize key points and maintain the audience's attention. By varying the pitch, tone, and volume of their voice, instructors can highlight important information, signifying its relevance or urgency. This modulation helps to engage the students, making the delivery more dynamic and facilitating better retention of the material being presented. When an instructor raises their voice slightly or changes the inflection, it can signal to the audience that what they're about to hear is significant, prompting students to pay closer attention and absorb the lesson's main concepts.

In contrast, other options do not align with the primary educational objective of using voice fluctuations. Indicating displeasure or sarcasm would not contribute positively to the learning environment and could create a negative atmosphere. Similarly, using fluctuations to avoid questions would not be conducive to effective teaching and would undermine the instructor's role in fostering learning and open communication.