The correct answer is C) sheath-screw davit. A sheath-screw davit requires turning a crank in order to swing the lifeboat out over the ship's side. This is because the sheath-screw davit uses a threaded rod or screw mechanism to control the outboard movement of the davit arm, which must be manually operated. In contrast, the other davit types listed (radial, bruckner, and gravity) use different mechanisms that do not require manually turning a crank. The requirement for a crank-operated sheath-screw davit is specified in regulations governing the design and equipment of lifeboats and launching appliances on merchant vessels.