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.
The answer is B) It suggest a tone of worry. The reason is because the phrase "knitting their cheeks" means the that the llamas were moving their jaws. This along with the fact that they were contemplating the writer suggests that the llamas are unsure of the writer.