The literary device used in the title of Oscar Wilde's play "The importance of being earnest" is pun.
What is pun?
Pun refers to a literary device that involves play of words, generally the homophones, that are the words with same pronunciation and different meanings.
In the title of the play, The importance of being earnest, the author uses the literary device pun with the words importance and earnest.
Therefore correct option is C.
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