Answer: D.To love that well which thou must leave ere long.
Step-by-step explanation:
You didn't attach the excerpt and the the options but I searched online and found them.
The excerpt has been attached. The lines from the poem best develops the authors theme that people should love and be loved while they're still alive is "To love that well which thou must leave ere long".
Other options such as "In me thou seest the twilight of such day" doesn't follow the theme. Therefore, the correct option is D.