In this problem the force is constant, set by the limit that the person must survive. The time of impact is different for the different surfaces. Impact on concrete has the shortest time of impact, so for a fixed force has the smallest survivable impulse and therefore the smallest change in momentum. Therefore the fall must be from the lowest height so the velocity just before the impact is the smallest. Water has the longest time of impact, so the largest survivable impulse so it can have the largest change in momentum and therefore a fall from the highest point.
Soil is between water and concrete in each step of the reasoning.
The velocity is given as the same. Thus momentum, which is mass times velocity, doesn't change. This can situation can be viewed two ways. First, there is an outside force, the ground, acting on the apple so momentum is not conserved. The outside force gives an impulse which changes the momentum.