Answer:
The literary device used in this line is a epigram
Step-by-step explanation:
Epigram is a brief poetic composition that expresses a single main thought as is the case of the line of Act II of The Importance of Being Earnest shown in the question.
The Epigram was created in Classical Greece and, as the meaning of the term indicates, was an inscription that was placed on an object - a statue or a tomb, for example. The epigrams on the tombs formed a class apart and were called Epitaphs or Epicuries, designating an ingenious poem that had the characteristic to be brief, to be able to pass by label or inscription.