How does maximum efficiency of turbojet engines relate to altitude?

The maximum efficiency of turbojet engines is largely influenced by the aircraft's operating altitude due to the characteristics of air density and temperature. As altitude increases, air density decreases, which affects the amount of oxygen available for combustion in the engine. Turbojet engines are designed to operate optimally in thinner air, where they experience reduced drag and can achieve higher speeds.

At altitudes around 35,000 feet and above, turbojet engines can operate in conditions that allow for higher thermal efficiency and better fuel economy, as the engines can produce more thrust with a more favorable fuel-to-air ratio. Additionally, the reduced drag at these higher altitudes contributes to greater overall efficiency during cruise flight.

In contrast, operating below 30,000 feet often results in less efficient performance, primarily due to denser air, which increases drag and can hinder the engine's ability to produce optimal thrust. Thus, the correct answer highlights that maximum efficiency of turbojet engines is commonly reached at or above 35,000 feet.