Answer:
D. She offered him a hundred-watt smile.
Step-by-step explanation:
In literature, a metaphor refers to a figure of speech in which the person states an IMPLICIT relationship or comparison between two things that, technically and literally are not related but that share some characteristics. And it is because of these characteristics that the metaphor is created.
In these examples, both A and C state in a explicit way "you are like" or "like unto", therefore, the comparison they are making is explicit.
Option B is not really making a comparison between two different things.
Finally, option D says "She offered him a hundred-watt smile", this sentence is establishing a relation between a smile and a hundred-watt light. The comparison is made in an implicit way and it is made because the smile of the girl is so bright that it shines a a hundred-watt light. Therefore, this would be an example of a metaphor.