The correct answer is option 4 - zero gravity.

When a spaceship is orbiting the Earth, the force of gravity that pulls objects towards the Earth is still present. However, the feeling of weightlessness is experienced because both the spaceship and everything inside it are falling towards the Earth with the same acceleration. This is known as free fall.

In this situation, the astronauts and objects inside the spaceship are not pushing against any solid surface, and hence, there is no normal force acting on them. The absence of a normal force results in a sensation of weightlessness.

The term "zero gravity" is often used to describe this condition, although it is not completely accurate. In reality, gravity is still present, but the sensation of weight is nullified due to the free fall motion. This can be observed in other situations as well, like when jumping off a diving board or when skydiving.

Therefore, while orbiting the Earth in a spaceship, weightlessness is experienced as a result of the absence of a normal force due to free fall, rather than zero gravity.