What kept the box of books from moving when Karol suddenly stopped the rolling cart?

The correct answer is inertia. Inertia is the property of an object to resist changes in its state of motion. When Karol stopped the rolling cart suddenly, the box of books inside it had a tendency to continue moving forward due to its own inertia. It was the inertia of the box that kept it from sliding out of the cart immediately.

Gravity, while it does act on the box by pulling it downward, does not directly influence its horizontal motion in this scenario. Momentum refers to the mass of an object multiplied by its velocity, but it doesn't describe the reason the box remained stationary when the cart stopped. Friction involves the resistance between the box and the surface of the cart, which can contribute to the box staying in place, but it is predominantly inertia that explains why the box continued to resist the change in motion when the cart abruptly halted.