Which of the following jet engines is NOT typically categorized under turbojet engines?

The turboprop engine is distinct from turbojet engines due to its design and function, which focuses on propelling an aircraft using a propeller rather than relying solely on jet thrust. Turbojets operate by compressing air, mixing it with fuel, and then igniting the mixture to generate thrust through high-speed exhaust. In contrast, turboprop engines combine a gas turbine engine with a propeller, making them particularly efficient at lower speeds and altitudes, where aerodynamic efficiency is critical.

In this distinction, turboprop engines rely on their propellers to produce the majority of the thrust, while turbojets and the other types listed primarily produce thrust via jet propulsion. The scramjet, meanwhile, is a form of air-breathing engine that operates at hypersonic speeds, also falling outside the typical turbojet categorization. Thus, recognizing these differences highlights why the turboprop is the correct response to the question.