The correct answer is C) proportional plus reset plus rate action. The main propulsion, boiler feed water regulating system commonly used aboard ships typically utilizes a triple element control system that combines proportional, reset (integral), and rate (derivative) actions. This type of sophisticated control system is designed to precisely regulate the boiler feed water flow to maintain the desired steam pressure and level in the boiler, which is critical for the efficient and safe operation of the main propulsion system. The other options are incorrect because: A) two-position differential gap action is a simpler on-off type of control system, B) proportional action alone is not sufficient for the precise control required, and D) proportional plus reset action, while more advanced than A, still lacks the rate component necessary for the full triple element control.