Checking the options

• A. when the acceleration of the object becomes negative
  ? Wrong. Acceleration doesn’t turn negative (upwards); it reduces to zero.

• B. when the acceleration of the object is equal to g
  ? Wrong. That’s true only at the start, before air resistance builds up.

• C. when the air resistance equals the weight of the object
  ? Correct. That means resultant force = 0, so acceleration = 0, hence terminal velocity.

• D. when the air resistance is greater than the weight of the object
  ? Wrong. If air resistance > weight, the resultant force would act upwards and the object would actually decelerate.

Related Concept:

When an object falls through the air:

1. At the start:

   • Its velocity is small.

   • Air resistance is negligible.

   • So the only significant force is weight (W = mg) acting downwards.

   • The resultant force = weight, so the object accelerates downwards at almost g.

2. As velocity increases:

   • Air resistance increases.

   • Resultant force = weight – air resistance.

   • Acceleration decreases (less than g now).

3. Eventually:

   • Air resistance grows until it becomes equal to the weight.

   • At this point, resultant force = 0.

   • If resultant force = 0, acceleration = 0.

   • So the object continues falling at constant velocity (this is terminal velocity)