Explanation:

Given data:

Initial velocity, u = 0

Final velocity, v = 20 m/s (given that it reaches the ground with a velocity of 20 m/s)

Distance covered, s = 100 m

As we know, the equation of motion for uniform acceleration is:

s = ut + (1/2)at²

where s is the distance covered, u is the initial velocity, a is the acceleration and t is the time taken.

On substituting the given values in the above equation, we get:

100 = 0 + (1/2)at²

On solving for a, we get:

a = (2s) / t²

Now, as the object is dropped from rest, its initial velocity, u = 0. Thus, the time taken to cover the distance, s = 100 m can be given by:

s = (1/2)at²

100 = (1/2)gt²

On solving for t, we get:

t = √(200/g)

On substituting this value of t in the equation of acceleration, we get:

a = (2s / (200/g))

On simplifying this equation, we get:

a = 10g

Therefore, the acceleration due to gravity is 10 m/sec².

Thus, option A is the correct answer.