Answer :
I would go with A.
It is definitely not length because that has nothing to do with it, the subject shouldn't affect formal or informal text. Quality is the closest but I think it is incorrect because a story that is written informally can have high quality.
It is definitely not length because that has nothing to do with it, the subject shouldn't affect formal or informal text. Quality is the closest but I think it is incorrect because a story that is written informally can have high quality.