Answer:
Part a)

Part b)

Part c)

Step-by-step explanation:
Part a)
Distance between two balls when it is dropped is given as

relative velocity of two balls initially at time of drop

relative acceleration of two balls is given as

now we have


Part b)
now the maximum height that we can use here is H
then we can say time to reach the ball back on the ground which is projected from ground is

Now if other ball just reach the ground

so we have


Part c)
If collision occurs at the top position of ball projected from the ground
then at the highest position

this must be time of collision so we have

