The correct answer is A) Controllable-pitch propeller. In an electricity generating plant with shaft-driven generators, a controllable-pitch propeller is generally used for main propulsion. This is because the controllable-pitch propeller allows for precise control of the vessel's speed and maneuverability, which is essential for efficient power generation and distribution. The ability to adjust the pitch of the propeller blades enables the operator to optimize the propeller's efficiency and match the power output of the generators. The other options are not typically used in this application. Fixed-pitch propellers do not offer the same level of control, tandem propellers are more commonly used for high-speed vessels, and detachable-blade propellers are generally not found in electricity generating plants.