Question

it is easier to lift a heavy stone under water than in air . explain

Answer 1

Because along with the force you provide, buoyant force is also working. As soon as it is in air no buoyant force is working so only one force is applied.

Answer 2

It is easier to lift a stone underwater than in air because underwater a force called buoyant force of upthrust applies in the objects in water in the upward direction, so If you get to lift a stone underwater of will be comparatively lighter than trying to lift it while I'm air.