The correct answer is spaceships.
The topic of science fiction stories are events that might be years, if not centuries ahead of us. Such stories contain some things that don't exist now, or that are completely different from what they are like in the stories. You can take Star Trek as an example, with various alien races, space ships, the exploration of space, etc.
With this in mind, the correct answer is definitely spaceships. Given that we already have helicopters, boats, horse carts, and cars, there's nothing fictional about those, but spaceships are still an unknown for us.