A man getting off a running bus falls forward due to:

According to the law of inertia of motion, the body resists any kind of change in the state of its motion.

If a man gets down from a running bus, then the man falls forward because the lower part of his body comes to rest on the ground but the upper part tends to continue to be in its motion due to inertia of motion and moves forward.

Therefore, 'Inertia of motion' is the correct response.