Answer:
The correct answer is option B.
Step-by-step explanation:
The author uses the simile to compare the boat riding the waves, with two gray horses.
If we pay attention to the full quote, we can see it more clearly:
"So narrow is the lane that as she enters the water rises on the shingle banks and flows in waves on either side of her like two gray horses with white manes that canter slowly along, a solemn escort, until the channel between the islands is passed."
The simile, also called comparison, is a rhetorical figure that serves as a means to establish a relationship between a real element and an imaginary or figurative one. For example: It was cold as an iceberg.
The simile is an element that is quite easy to identify, because unlike what happens in other rhetorical figures, such as metaphor, in similes both elements are named and so is the link that unites these two elements.