Final answer:
The line "So long as men can breathe or eyes can see" from Shakespeare's Sonnet 18 suggests that the poet's words immortalize his love's beauty for as long as people live and can see.
Step-by-step explanation:
In the context of Shakespeare's Sonnet 18, the line "So long as men can breathe or eyes can see" refers to the enduring nature of the poet's written words about his love. It is stating that as long as people live and are capable of seeing, the poet's words will continue to exist, thus immortalizing the beauty of the subject he writes about. This line is part of the sonnet's final couplet, which serves as a concluding statement, accentuating the theme that the poetic word gives eternal life to the subject's beauty. The poet contrasts the transient nature of beauty and human life with the timeless nature of his verse. Therefore, the correct answer to the question is (a) The enduring nature of the poet's love.