Answer: D. Deep, low, happy sigh
Step-by-step explanation:
Imagery happens when the writer employs words with the purpose of appealing to the reader's senses. The reader is, thus, able to imagine a certain scenery, action, smell, or even a sound from the way it is described in the text.
In the passage we are analyzing here, the author chose to describe the sigh heard from the crowd. It is not a mere sigh, it is a "deep, low, happy" one. The reader can most vividly imagine such sigh after reading that description.