The correct answer is D) proportional plus reset action. A pneumatic dual element, main propulsion, boiler feed water regulating system commonly used aboard ship utilizes proportional plus reset action to maintain the desired water level in the boiler. This type of control system combines proportional action, which provides a control output proportional to the error between the desired and actual water level, with reset action, which slowly adjusts the control output to eliminate any steady-state error and maintain the target water level. The other options, such as two-position differential action or on-off reset action, do not provide the precise control and error elimination required for this type of boiler feed water regulation system.