Answer: I would contend that the right answer is actually the B. "My vegetable love should grow..."
Explanation: Just to elaborate a little on the answer, it can be added that although options C and D are indeed examples of figurative language (hyperbole and simile, respectively), I believe that option B, which is an example of a metaphor, is actually the one that best explore the theme of this poem—an invitation to enjoy the moment, to live and to love, the so-called <em>carpe diem</em> theme. The speaker of this poem is addressing his coy mistress, and he does not hesitate to be frank and straightforward with her, since their time is limited and their desire very intense. There are many interpretations of the possible meaning of the expression "my vegetable love," but it short it can be said that he is warning her that, if they had time, his slow vegetable love would grow slowly and amply (perhaps a reference to chastity), but they don't have it, so he urges her to start loving each other with passion and without regrets.