Final answer:
In Shakespeare's Sonnet 18, the line 'But thy eternal summer shall not fade' suggests the permanence of the subject's beauty.
Step-by-step explanation:
In Shakespeare's Sonnet 18, the line 'But thy eternal summer shall not fade' suggests b) Permanence of the subject's beauty. The speaker is comparing the subject of the poem to a summer's day, stating that their beauty is even lovelier and more enduring. By using the phrase 'eternal summer,' the speaker implies that the subject's beauty will never fade or diminish.