It's not a denotation. If it were, the earth would be cinder long forgotten millions of years ago.
It really isn't to reveal the importance of watching the sky. The word burn is used poetically. If the poem wanted you to watch the skies, it would have said so either directly or indirectly using poetic language. Nowhere are you being asked to watch the skies.
There is nothing in theory about the burn and as a consequence there is nothing critical in the tone.
That only leaves A but it is not a very good answer.
A <<<< answer.