The correct answer is C) Controllable-pitch propeller. In an electricity generating plant with shaft-driven generators, a controllable-pitch propeller is generally used for the main propulsion. This is because a controllable-pitch propeller allows for efficient and flexible control of the propeller thrust, which is important for matching the power output of the shaft-driven generators. The ability to adjust the pitch of the propeller blades enables the operator to optimize the propeller's performance and match the power demands of the electrical plant. The other options, such as tandem propellers, detachable-blade propellers, and fixed-pitch propellers, are not as well-suited for this application, as they lack the same level of thrust control and flexibility required for an electricity generating plant with shaft-driven generators.