Explanation:

- Option 1: Acceleration

Many think weightlessness comes from acceleration, but in orbit, both the spaceship and astronauts are actually "falling" together. There's acceleration due to gravity, but since everything inside the ship is falling at the same rate, you feel weightless.

- Option 2: Inertia

Inertia is the tendency of objects to keep moving at constant speed in a straight line unless acted upon. It plays a role in space travel but isn't the direct reason for weightlessness.

- Option 3: Zero gravity

Often called "zero gravity," but it's not truly zero—gravity is still present. The feeling of weightlessness comes from "free-fall." Both you and the spaceship are falling around the Earth together, giving the sensation of zero gravity.

- Option 4: Centre of gravity

Centre of gravity refers to the point where weight is balanced. It's not responsible for the sensation of weightlessness in orbit.

Correct Answer: Option 3 – Zero gravity