The correct answer is D. Her room is a pigsty.
Figures of speech are words and expressions we use in order to convey some idea; however, these words and expressions are usually not supposed to be taken literally - rather, they have symbolic meanings. Options A, B, and C are using the literal meanings of trash, junk, and messiness - meaning there are no figures of speech there.
However, if you take a look at option D, you will see a figure of speech - to be precise, it is a metaphor. The narrator is comparing her room to a pigsty - it is not literally a pigsty, but rather just resembles one.
