C is considered to be a sentence fragment because it looks like a sentence but it isn't.
The boys built a fort in the forest
The answer you picked is correct
When you analyze a structure of a writing or some articles, first of all you must consider the every details that could make the writings as a whole and ask what composes it and how is this structured. The best answer to this question is this sentences. "What is the source of details-personal experience, experiments, scholarly source, or facts?"
