Answer:
The magnitude of its acceleration in the water is 1.5*10² meters/second²
Step-by-step explanation:
Acceleration tells us how the speed changes. Therefore, it relates the changes in speed with the time it takes to produce. So

The speed change is ending speed - starting speed. So in this case, the speed change is 0 - (43 m/s) = -43 m/s . [ You know that the hailstone travels with a speed of 43 meters / second (initial speed) and stops (final speed = zero) 0.28 seconds after hitting the water.]
And time spent changing speed is 0.28 seconds.
Considering the speed change module, that is the number regardless of the sign:

acceleration=153.57 m/s²≅1.5*10² meters/second²
The magnitude of its acceleration in the water is 1.5*10² meters/second²