The momentum of a body is given by the equation momentum = mass * velocity. In this question, we are given the initial and final momentum of the body, as well as the duration of the force applied.

To calculate the magnitude of the force, we can use the equation force = change in momentum / change in time.

Given that the initial momentum is 10 g cm/s and the final momentum is 40 g cm/s, the change in momentum is 40 g cm/s - 10 g cm/s = 30 g cm/s.

The duration of the force applied is given as 3 seconds.

Using the equation force = change in momentum / change in time, we can substitute the values we have to find the magnitude of the force.

force = 30 g cm/s / 3 s = 10 g cm/s^2

Since force is usually measured in dynes or newtons, we need to convert the units.

1 newton = 100000 dynes

Therefore, the magnitude of the force F is 10 g cm/s^2, which is equivalent to 10 dynes.

Therefore, option 1 "10 dynes" is the correct answer.