A. Modern novelists often expand to other genres, such as screenwriting.
Broad and generic words like "things" and "stuff" are too vague for formal writing. They make the author's points weak and not well thought out. Options B, C, and D use these vague words so they do not have precise wording. Option A uses the word genres instead of "things" or "stuff".
The sentence which is the most subjective is D. My mother had to be the best cook in the world. It is subjective because that is your own opinion - the other examples are just facts.
