The correct answer is A) Hydraulic motor. Marine gas turbine engines commonly use hydraulic starter motors to initiate the engine's rotation during startup. Hydraulic motors are well-suited for this application due to their high torque output and ability to provide the necessary cranking power to start the turbine engine. Other options like AC or DC electric motors would not be as suitable for this high-power starting requirement. The other answer choices are incorrect because AC synchronous and induction motors are not typically used to start gas turbine engines, and a DC series wound electric motor would not have the torque capacity needed for this application.