Answer: So long as men can breathe or eyes can see, So long lives this, and this gives life to thee.
Step-by-step explanation:
A couplet refers to successive lines in a poem, play, drama or otherwise that rhyme and have the same length. Couplets can be Open or Closed with the former referring to couplets which exist in multiple lines as opposed to the latter where they would be in separate sentences.
In Sonnet 18, the answer above is the couplet as the lines rhyme by the use of the words 'see' and 'thee'.