Which is the best definition of metaphor?

a.
a figure of speech that makes a comparison using the words like or as; for example, "i'm as hungry as a horse"

b.
a figure of speech that states a comparison between two unlike things; for example, “i’m the dew on the rose”

c.
a figure of speech that makes a comparison using numbers; for example, "i am one of two artists here today"

d.
a figure of speech that states a comparison between the speaker and another person; for example, "i am older than sheila"