Calculation of Change in Momentum:
The change in momentum of an object is given by the product of the force applied and the time interval over which it is applied. In this case, we are given that a force of 20 N is applied to the body for a time interval of 10 s.

- Therefore, the change in momentum (Δp) can be calculated using the formula:
Δp = F * t
Δp = 20 N * 10 s
Δp = 200 N.s or 200 kg.m/s

Calculation of Magnitude of Change in Momentum in One Second:
To find the magnitude of the change in momentum in one second, we need to divide the total change in momentum by the time interval of one second.

- Given that the total change in momentum is 200 kg.m/s, we can calculate the magnitude of change in one second as follows:
Magnitude of Change in One Second = Δp / t
Magnitude of Change in One Second = 200 kg.m/s / 10 s
Magnitude of Change in One Second = 20 kg.m/s

Explanation of Correct Answer:
The correct answer is option 'B', which states that the magnitude of change in momentum of the body in one second is 20 kg.m/s.

- This is because the magnitude of change in momentum is equal to the force applied, and in this case, a force of 20 N is applied to the body.
- Since the force is constant, the magnitude of change in momentum remains the same throughout the 10-second interval.
- Therefore, the magnitude of change in momentum in one second is also 20 kg.m/s.

Conclusion:
The magnitude of change in momentum of the body in one second is 20 kg.m/s. This is calculated by dividing the total change in momentum (200 kg.m/s) by the time interval (10 s). It is important to note that the magnitude of change in momentum is equal to the force applied, which in this case is 20 N.