The correct answer is answer C ("Moon").
In The Red Badge of Courage, Henry Fleming is a private of the army who escapes his duties and the field of battle. The red badge symbolizes the wound he hopes would be inflicted upon him as a punishment for his cowardice, since it makes him feel deeply ashamed to have fled war.
His job was to be the standard-bearer, which means to carry the flag in the fields, hence its importance to the story.
The sun is also utilized both metaphorically and figuratively in several occasions during the narration. In fact there's an entire book written about the role that the sun plays in the story.
However, there are no mentions of the moon used as any kind of significant symbol in this book.
Hope this helps!