A figure of speech in which a word or phrase is applied to an object or action to which it is not literally applicable is metaphor.
Option C is correct option as in this line here is implied comparison Perfect note of thoughts .
Thoughts are compared with perfect note, so not a simile although both simile and metaphor are same but a slight difference of words in simile words "like" or as are use to make comparison.
Other are options also wrong as all other figures of speech makes different meaning.
In this whole poem the central idea is about from heart to soul that the life is uncertain, never remain thoughtful for the happiness and all other emotions because they are temporary.