This is an example of inertia - the body keeps it's energy because there is no force applied to it. When we try to stop it's motion, it resists. A man is not rigidly attached to the bus, so he keeps moving forward, at least until he hits the front window from inside. Answer is D.
Force = mass * acceleration
F = ma
Given m = 10 kg, F = 30 N;
F = ma
30 = 10a
Solving for a:
a = 3 m/s^2
The acceleration is 3 meters per second squared.
I believe the answer would be 7.5 m/s^2