A1-kg stone and a 10-kg stone have the same acceleration in free fall because

The free fall of a body is defined as the motion of a body dropped from a certain height under the action of gravity only.that gravity  operates between the body and the the earth.during the free fall ,all the bodies have the same acceleration i.e the acceleration due to gravity[g]

as per the question two bodies having weight 1 kg and 10 kg are dropped from a certain height.let the air resistance is neglected.

for one 1 kg object-

as the body is falling under gravity only ,hence the force of gravity between body and the earth is - newton  [1]

as per newtons second law -  [2]

comparing 1 and 2 we get-

the acceleration is found to be a=g

similarly for 10 kg object the force of gravity is 10g newton

like before the acceleration is found to be same i.e g

hence all the  free falling objects have the same acceleration irrespective of the mass of the objects.