The day shifted slowly to dusk, with a pale sliver of moon casting an eerie glow. Which of the following correctly describes this text?

Yep, it was definitely the setting.

the setting is like a background, or where you are at the moment,

while a backstory is someone's past, a conflict is a struggle against an obstacle, like the person's thoughts, or other people, and the plot is what the story is about.

I think the answer you're looking for is "setting".