The correct answer is D) Controllable-pitch propeller. Tunnel bow thrusters used in electric motor driven, transverse configurations typically employ a controllable-pitch propeller design. This allows the starting current of the single speed drive motor to be limited by adjusting the pitch of the propeller blades, reducing the initial load on the motor during startup. The variable pitch capability of the propeller is a key feature that makes it the preferred propulsor type for this application, as opposed to fixed-pitch, detachable-blade, or cycloidal propellers which do not provide the same ability to control starting current.