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

So innocent arch, so cunningly simple.

Oxymoron

The correct answer is OPTION 4 - Oxymoron.

An oxymoron is a figure of speech in which two contradictory terms are used together. In the expression "So innocent arch, so cunningly simple," the words "innocent" and "cunningly" are contradictory. This creates a sense of paradox, which is appropriate because the speaker is describing someone who is both naive and manipulative at the same time.

The other answer choices are incorrect. Pun is a play on words, simile is a comparison between two things using the words "like" or "as," and metaphor is a comparison between two things without using the words "like" or "as." None of these devices are used in the expression "So innocent arch, so cunningly simple."