The is the underlying message or idea in a literary work. A.
summary
B.
lesson
C.
theme
D.
subject

I think the correct answer from the choices listed above is option B. The theme that is emphasized in the excerpt from "Ode on a Grecian Urn" by John Keats is that the unfulfilled desires and dreams seem sweeter. It is can be seen from the lines:

Heard melodies are sweet, but those unheard  

Are sweeter; therefore, ye soft pipes, play on;

Not to the sensual ear, but, more endear'd,

Hope this helps you! :D