The correct answer is C. inertia of the passengers.
Inertia is the tendency of an object to resist any change in its motion. When a bus stops suddenly, the passengers are still moving forward because of their inertia. The friction between the passengers and the bus seats helps to slow them down, but it is not enough to stop them completely. As a result, the passengers are pushed forward when the bus stops.
Option A is incorrect because the friction between the earth and the bus does not cause the passengers to be pushed forward. The friction between the earth and the bus helps to slow the bus down, but it does not affect the motion of the passengers.
Option B is incorrect because the friction between the passengers and the earth does not cause the passengers to be pushed forward. The friction between the passengers and the earth helps to slow the passengers down, but it does not affect the motion of the bus.
Option D is incorrect because the inertia of the bus does not cause the passengers to be pushed forward. The inertia of the bus helps to keep the bus moving forward, but it does not affect the motion of the passengers.