Which of these is an example of a metaphor?

a she is a rose without a thorn.
b my love is like a red, red rose.
c the flames made darkness visible.
d the wind was awake and disturbed the lake.

A. She is a rose without a thorn.

Explanation:

Metaphors are figures of speech that assert that one element (person, animal, thing, quality, event, nature) is similar to another element. They are alike similes, but while similes use words such as "like" or "as" to compare, metaphors do not.  

Option A is an example of metaphor because it asserts that "she" is similar or equal to "a rose without a thorn", figuratively speaking.

c, his tragic flaw to cause his downfall