What literary device is most prominent in this excerpt from Moore's poem, "Silence"?

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" ... not in silence, but restraint."

Connotation.

Explanation:

"Silence" poem is written by Marianne Moore. The poet speaks about the advice that the speaker's father has bestowed upon him/her in childhood. The advice given by the father was to be silent.

The literary device used in this line is connotation.

Connotation is the literary device which is used by the writer to suggest different meaning of the word than its literal meaning.

The word silence means willfully not expressing what one feels but restraint means a forceful suppression by which one is kept from speaking anything.

So, in the conclusive paragraph of the poem, the speaker is trying to say that her father is not teaching her to be silent, i. e., willfully not speaking but restraining her from speaking, meaning that even if she wants to speak she should keep silent.

So, the correct answer is connotation.