Which of the following is a physical characteristic of jet engines?

Jet engines are characterized by their use of rotating components, specifically compressors and turbines, which play a vital role in their operation. In a typical jet engine, air is drawn in and compressed by the rotating compressors, which increases the air's pressure and temperature. This compressed air is then mixed with fuel and ignited in the combustion chamber, leading to rapid expansion of gases. These gases flow through the turbine, causing it to rotate, and are eventually expelled out of the engine at high speed. This high-speed outflow of gases generates thrust according to Newton’s third law of motion, propelling the aircraft forward.

The other options present characteristics that do not align with the typical design and function of jet engines. For instance, jet engines do not employ fixed propellers for thrust generation, as they rely on the principles of jet propulsion rather than rotational thrust from propellers. Additionally, jet engines are not powered solely by electric means; they primarily function on the combustion of fuel. Lastly, while pilots do manage and adjust thrust, modern jet engines, especially in commercial aviation, are controlled by sophisticated automated systems that reduce the need for manual input, allowing for greater efficiency and safety.