Work is done on a body only when-

Answer: Option C

Work is defined in physics as the product of force and displacement in the direction of the force.

This means that work is only said to be done when a force causes displacement of a body.

If there is no displacement, even if a force is applied, no work is done in the physics sense.

Option A is incorrect because energy gain alone doesn't confirm that mechanical work has occurred—it must involve displacement.

Option B is misleading; forces may act on a body, but without displacement, no work is done.

Option D is close, but too vague—movement alone isn't enough; it must be displacement caused by a force.

Therefore, Option C is correct because displacement is the essential condition for work to be done on a body.