What is the key feature that differentiates turboprop engines from turbojet engines?

The key feature that differentiates turboprop engines from turbojet engines is the design and function associated with propulsion. Turboprop engines are characterized by their incorporation of a propeller driven by a gas turbine, which allows them to convert a portion of the turbine's energy into thrust more effectively at lower speeds and altitudes. The presence of more turbine stages and a geared prop system enables turboprop engines to optimize this energy conversion, making them especially efficient for regional and short-haul flights.

In contrast, turbojet engines are designed primarily for high-speed flight, utilizing jet propulsion and producing thrust through the expulsion of high-speed exhaust gases. This design feature is what primarily distinguishes turboprops from turbojets.

The other options do not accurately capture the fundamental difference between turboprop and turbojet engines. While turboprops indeed have advantages in fuel efficiency at lower altitudes compared to turbojets, this efficiency alone does not define their operational characteristics. Additionally, turboprops are not exclusively used for military aircraft, as they are widely utilized in commercial aviation as well. Lastly, turboprop engines do not operate at higher speeds than turbojets; rather, they are optimized for lower-speed operations.