Tonya’s psychology teacher played some backwards music to the class, and they were not able to make out any words or phrases in the selection. when the teacher told them to listen for the words "the rat ate the cat" when she played it again, most of the class now heard the words. which principle explains this experience

Perceptual set.

Explanation:

The psychologist Allport, who studied perception, defined perceptual set as the tendency of a person to perceive things in a certain way, this happens because of a predisposition that the person has, this situation is a bias and can happen in different senses.

In the question, Tonya's psychology teacher played some backwards music and the class doesn't recognize any words of phrases. However, when the teacher tells them to pay attention to listen for the words "The rat ate the cat" most of the class heard the words because they were predisposed by what the teacher told  them and therefore they could perceive the sounds in a different way. Therefore, this is an example of perceptual set.