Read the sentence.

marissa flopped onto the raft and lay on her little island, warming her back in the sun.

which figure of speech, or literary device, does the author use in the sentence?

Implied metaphor.

Explanation:

Implied metaphor is a type of metaphor that makes a comparison of two things without mentioning one thing.  

"Marissa flopped onto the raft and lay on her little island, warming her back in the sun."

Here, the raft is metaphorically imagined as a "little island". Though it is never possible to even think of a raft as an island, with the use of implied metaphor, the author successfully managed to bring home the main point of the sentence. The raft represents the "little island" that Marissa can have on her own and enjoy.

a. the scene is structured as a dialogue between two characters in their own informal language.