Final answer:
In Shakespeare's 'Sonnet 18,' the 'this' is the sonnet itself, which memorializes the subject's beauty, granting it immortality as long as the poem is read.
Step-by-step explanation:
The 'this' referred to in the last line of Shakespeare's "Sonnet 18" ('So long lives this, and this gives life to thee.') is the sonnet itself. Shakespeare is discussing the power of poetry to confer immortality upon the subject. Through the couplet, the poet claims that as long as people breathe and can see, the subject's beauty and essence will live on in the enduring lines of the poem. Essentially, the sonnet functions as a means for the subject's eternal summer or beauty to not fade, despite the inevitable passage of time and mortality.