What defines a turbo jet engine?

A turbojet engine is characterized by its simple design and low frontal area, which makes option B the most accurate choice. The turbojet operates by compressing incoming air, mixing it with fuel, and igniting it. This process produces high-speed exhaust gases, creating thrust. The streamlined shape of the engine contributes to its aerodynamic efficiency, making it suitable for high-speed flight.

While turbojets can generate significant thrust, they are not limited to being categorized primarily by their thrust output, as in option A. The statement about complexity and cost in option C may apply to certain engine designs but does not capture the fundamental defining characteristics of all turbojet engines. Option D is incorrect because turbojet engines typically have a higher power output compared to other engine types, such as turbofan engines, particularly at high speeds.

Thus, the simplicity in design and reduced frontal area are what truly define a turbojet engine, aligning with the choice selected.