The correct answer is C.
A dystopia is a "futuristic, imagined universe in which oppressive societal control and the illusion of a perfect society are maintained through corporate, technological, moral, or totalitarian control". It is the opposity of an utopia.
A world that has plunged into chaos because of the government removing the right to electricity from rebel communities is an example of a scenario for a dystopian story, since it shows how the oppresion of the government leads to a disastrous change on society's functioning. And it also shows how this scenario is maintained by the government's totalitarian control.
The rest of the answers, in which people disappear, aliens replace teenagers and a genius boy is discovered living in a library cellar, would make for good sci-fi scenarios rahter than dystopian societies.
