The correct answer is C. spaceships.
Science fiction is a literary genre that deals with the things that we don't really have at the moment, but can imagine them - things such as spaceships, sentient robots, space battles, space explorations, space travel, distant planets and alien species, etc.
Given that we already have and use helicopters, boats, horse carts, and cars, we can safely deduce that these modes of transportation are not science fiction because they are real. On the other hand, we don't have spaceships, which makes them an element of science fiction stories.