Final answer:
The sentence 'The sunburn was a fire that spread across my back.' contains a metaphor as it compares a sunburn directly to a fire without using 'like' or 'as'. Therefore the correct answer is Option B.
Step-by-step explanation:
The sentence uses a metaphor. A metaphor is a figure of speech that compares two unrelated things by saying one thing is another. In this case, the sunburn is compared to a fire that spread across the speaker's back. The use of 'was' in the sentence indicates that this comparison is being made.