When equals the force of gravity on a falling object, the object reaches terminal velocity.

The correct answer to the question is : Air resistance.

EXPLANATION:

Before coming into any conclusion, first we have to understand the terminal velocity.

An object is said to be moving with terminal velocity if the net force acting on the object is zero i.e the body is non accelerating.

As per the question, the object is falling under gravity. The force of gravity acts in vertical downward direction.

Except gravity, the object is also under air resistance. It is the force which opposes the motion of the object, and acts in vertical upward direction.

If the air resistance becomes equal to force of gravity, then the net force acting on the object is zero . It is so because two forces act in opposite direction.

In case of liquids, the resistance force is called as viscosity.

Hence, the correct answer to the question is air resistance.