The only force that can act on an object in free fall is .

Before going to answer this question first we have to understand free fall.

The motion of  body is said to be free fall when it is acted upon by the force of gravity only.There is no external force that acts on the body.

we know that every object in the universe attracts every other objects with a force called gravitational force.Hence, whenever a body falls,there will be force of attraction between body and  earth. This attractive force is called force of gravity by which  earth  attracts every objects towards its centre with an constant acceleration called acceleration due to gravity.

Let us consider a ball which is thrown to the sky with a velocity V.Let the air resistance is neglected.The ball will go high up ,reaches it's maximum height where its velocity becomes zero. The force of gravity acts on the body in vertical downward direction,so its velocity decreases at a rate of 9.8 m/s^2.

After reaching its maximum height the ball will start falling downward.Each time the velocity of the body increases at a rate of 9.8 m/s^2. That acceleration produced during free fall is nothing else except the acceleration due to gravity. Hence the force of gravity that acts on the body will be mass times g where g is the acceleration due to gravity.

Mathematically  F= mass ×acceleration due to gravity

                             = mg

Hence the only force that can act on the body during free fall is force of gravity.