Which line from act ii of the importance of being earnest is an epigram?

The correct answer is - "It is always painful to part from people whom one has known for a very brief space of time."
An epigram is a statement, which is usually short, clever, and sometimes funny. It usually contains some general truths about the world, but stated in a skillful manner. In the case of the play above, the epigram is also a bit ironic, given that we aren't really close to people we've just met. 

The step a mother is most likely motivated by is their child!