Final answer:
The third quatrain of Shakespeare's 'Sonnet 18' conveys the idea that the beloved's beauty will be immortalized through the poem, outlasting physical decay and death.
Step-by-step explanation:
The central idea of the third quatrain in Shakespeare's Sonnet 18 is that the beauty and essence of the beloved will achieve immortality through the enduring power of poetry.
These lines express the poet's belief that although physical beauty is transient, the beloved will continue to live on in the timeless verses of the poem. This idea is encapsulated in the lines, 'But thy eternal summer shall not fade, Nor lose possession of that fair thou ow'st, Nor shall death brag thou wander'st in his shade, When in eternal lines to time thou grow'st.' Therefore, the statement that best expresses the central idea of this quatrain is: Your beauty will never fade, because you will live forever in the lines of this poem.