Which is the best definition of metaphor?

a.
an unusual comparison that is meant to be funny; for example, "my uncle looks just like a sweet potato"

b.
a comparison between two things that are closely related; for example, "gorillas are larger than chimpanzees"

c.
a direct comparison between two unlike things; for example, "my tongue is sandpaper"

d.
an indirect comparison that uses the words like or as; for example, "you're as pretty as a picture"