Which sentence uses an idiom? a. the students were all ears while the teacher read the story. b. the students agreed that the story was full of adventure and mystery. c. the students thought the story was the best one they had heard. d. none of the students spoke while the teacher read the story.

A) The students were all ears while the teacher read the story.

Let's break it down:

A. The students were all ears while the teacher read the story.

This is an idiom. An idiom is a phrase or expression whose meaning can't be understood from the ordinary meanings of the words in it. For example, “Get off my back!” is an idiom meaning “Stop bothering me!”

B. The students agreed that the story was full of adventure and mystery.

This is not an idiom, it is describing the story.

C. The students thought the story was the best one they had heard.

This is not an idiom, it is describing the story.

D. None of the students spoke while the teacher read the story.

There is nothing close to an idiom in this option.