Answer: D. Give us ideas on how we should run things.
Dystopias are imaginary representations of society that are undesirable, frightening or unpleasant. It is the opposite of "utopia."
Dystopias are often set in the future, and they serve as a critique of our present world. Often, these societies are in such unpleasant conditions due to the actions of our present communities. By reading them, we understand the things we might be doing wrong and the consequences these actions could have. We also get ideas on how we could improve things.
Some examples of this kind of literature is George Orwell's 1984 (which highlights the dangers of governmental control) or Aldous Huxley's Brave New World.