The answer is: C. The author's position on significant themes.
The imagery is the way that the author create pictures that appeal to all of our senses in order to properly receives the full meaning that the poet attempts to transmit, the mood, on the other hand, are descriptions, words, tone, themes, diction use by the poet to evoke feelings on the reader, these two literary devices are used to convey in the reader the author position on significant themes.
Exclamation: If you're joining two sentences together you need a comma to do so. By placing a comma after "nation" and "then" you are making a run-on sentence.
