What happens to the engine's RPM when carburetor heat is applied?

When carburetor heat is applied, it introduces warm air into the carburetor. This warmer air is less dense than the cooler air that normally enters the carburetor. As a result, the fuel-air mixture becomes richer (more fuel relative to air) because there is less air to mix with the fuel.

This change in the fuel-air mixture affects the combustion process, initially leading to a decrease in engine RPM. The decrease occurs because the rich mixture can cause the engine to run less efficiently as it may not burn as completely as the correct mixture. After this initial drop in RPM, the engine will typically stabilize as the system adjusts and compensates for the change in mixture with increased fuel flow and adjustments in combustion efficiency.

Therefore, the correct answer is that the engine's RPM decreases and then stabilizes as a result of applying carburetor heat.