Answer: C. Your youthful beauty will not disappear.
Step-by-step explanation: A paraphrase is to express the meaning of a text using different words, usually with the function to clarify it. The given line from "Sonnet 18" by William Shakespeare "but thy eternal summer shall not fade" is best paraphrased with the sentence "your youthful beauty will not disappear" and it means that for the speaker, his loved one will be forever beautiful because of the love he feels for her.