That would be the voice because the voice is basically how the author writes. Like, their style. What types of words they use, the types/length of sentences they use, and the tone of how the author "talks" about certain things creates their voice. You can tell who wrote a piece of writing from their voice. Personally, I write a lot and I am friends with peole who enjoy writing just as much as me, and we all have our own writing voice, so you can tell our pieces of writing apart.
The reason it would not be the theme is because the theme is the message the story conveys, not how the author writes. The point of view is who's, well, point of view the story is written in! First person, third person omniscient, and third person limited. Conflict is a struggle between a charter and either another character, a natural force, or even themselves. So none of these really match up with what was stated.
