Final answer:
The sentence that uses a simile in the excerpt is "He kept his eyes fixed on her, marveling at the way her face changed with each turn of their talk, like a wheat-field under a summer breeze."
Step-by-step explanation:
The sentence in the excerpt from Ethan Frome by Edith Wharton that uses a simile is:
He kept his eyes fixed on her, marveling at the way her face changed with each turn of their talk, like a wheat-field under a summer breeze.
In this sentence, the comparison of the way her face changed to a wheat-field under a summer breeze creates a simile. The simile helps to emphasize the fluidity and beauty of her facial expressions.