What is the purpose of leaning the mixture at cruising altitude?

Leaning the mixture at cruising altitude is primarily done to optimize fuel efficiency in less dense air. As the aircraft ascends, the air density decreases. In such conditions, the engine does not require as much fuel to maintain optimal combustion efficiency. By leaning the mixture, a pilot adjusts the fuel-to-air ratio, allowing for more complete combustion of the fuel while reducing the amount of fuel injected into the engine. This results in improved fuel efficiency because the engine runs more effectively with less fuel in the less dense air, ultimately leading to longer endurance and reduced operational costs.

The other options highlight common misconceptions or incorrect reasons for leaning the mixture. Increasing fuel consumption would negate the benefits of fuel efficiency, while stabilizing engine temperature is more related to proper management of the engine rather than leaning alone. Reducing engine noise is not a direct effect of leaning; engine noise is affected by a variety of factors including engine design and airframe acoustics. Thus, the focus remains on how leaning optimizes fuel usage at higher altitudes.