Prove law of conservation of energy for an object thrown vertically upward .

Assume a case:

A ball thrown vertically upward with a certain initial KE will return to its original level with the same KE. When air resistance is a factor affecting the ball, will it return to its original level with the same, less, or more KE

Less KE. Air drag is a non-conservative force which dissipates mechanical energy as heat.

Does the answer contradict the law of energy conservation?
No, because the discrepancy energy ultimately increases thermal energy of the surrounding air. Since the kinetic energy remains same therefore the law of conservation of energy is not violated.