The final two lines of Shakespeare's "Shall I Compare Thee to a Summer's Day?" promises the subject of his sonnet

A. immortality.

B. an endless summer.

C. undying devotion.

D. unfading youth.

The answer is A. Immortality

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In "Shall I Compare Thee to a Summer's Day" Shakespeare promises the subject of his sonnet Immortality. He says that as long as men can breathe (which basically means as long as men exist on this planet) or as long as eyes can see, the feelings and the poem will exist, and it gives her life, thus making her practically immortal.