From the given options, select the Figure of Speech (Poetic Device) used in the expression:

As cool as cucumber

Simile

The correct answer is OPTION 4 - Simile

The figure of speech used in the expression "As cool as a cucumber" is simile.

A simile is a figure of speech in which two things are compared using the words "like" or "as." In this case, the person's calmness is being compared to the coolness of a cucumber. This creates a sense of the person's composure and lack of agitation.

The other answer choices are incorrect. Pun is a play on words. Metaphor is a comparison between two things without using the words "like" or "as." Oxymoron is a figure of speech in which two contradictory terms are used together. None of these devices are used in the expression "As cool as a cucumber.".