Answer: B) Metaphor.
Explanation: a metaphor is a figure of speech that consists in making a direct comparison between elements that aren't obviously related, in order to create an image in the reader's mind. The given sentence is a clear example of a metaphor that compares the fog to a wall, without using the words "like" or "as" (otherwise it would be a simile), so the correct answer is the corresponding to option B.
