D.To love that well which thou must leave ere long.
Shakespeare's sonnets often end with a concluding couplet. In this sonnet, the last line describes the theme of the sonnet. The speaker says, "to love that well which thou must leave ere long." Rewritten in more modern English, he says to love those things which you must leave before long. The idea here is that everything must leave at some point, that is how death works. But before you or other things leave, they should be loved. The last line of the sonnet best demonstrates this theme.