The correct answer is A. The nymph would accept the shepherd's love IF love lasted forever.
The nymph says "but could youth last," implying that growing old can diminish love. She also says that she would accept his love if joy had "no date." This implies that joy has an expiration date -- basically, the nymph is saying that nothing -- not happiness, not love -- lasts forever.
For this reason, she will not accept the shepherd's offer. If love lasted forever, she would; however, she does not believe love lasts forever.