The corrector answer is D. metaphor.
A metaphor is a comparison that does not use "like" or "as." (A simile, on the other hand, does use "like" or "as.") In the above example, Ross is saying that freedom of expression is LIKE an oil. Even though she is not using the word "like," she is still making a comparison between freedom and oil. Since she is not using the word like, the literary device is a metaphor.
