What safety mechanism prevents the elevator car from falling?

The safety brake or governor is the correct answer because it is specifically designed to prevent the elevator car from falling in the event of a malfunction or uncontrolled descent. The safety brake, often integrated with the governor, activates when the speed of the elevator exceeds a predetermined limit. This mechanism works by engaging the brake system, which effectively halts the movement of the elevator car, ensuring passenger safety.

The emergency stop button, while important for halting the elevator's motion when needed, does not serve as a fail-safe mechanism to prevent the car from falling in case of a serious issue. It is primarily used for immediate stops under normal operating conditions.

The overload sensor plays a critical role by detecting when the elevator is carrying more weight than its capacity, typically by stopping the elevator from moving if overloaded, but it does not directly prevent the car from falling.

The door locking mechanism ensures that the elevator doors are securely closed before the car can move, but it does not provide any protection against free fall. Its primary function is related to security and access rather than the safety of the car's movement itself.

Thus, the governor and safety brake are crucial components designed specifically to maintain the elevator's safety by preventing free fall.