Th final two lines of shakespeare’s “shall i compare them to a summers day? ” promises the subject of his sonnet —- and endless summer b. un fading youth c. immortality d. undying devotion.

I would guess D). "But thy eternal summer shall not fade" he is not talking about summer never ending. He is comparing her to summer and how his love will never end for her. He had said that Summer "lease hath all too short a date."
Shakespeare is saying that he loves her as much as a summers day. But since summer doesn't last forever, he compared her to an eternal summers day. His love never ends for her.

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