The correct answer is C) Sentinel valve. A sentinel valve is a type of safety valve found on the casing of a steam turbine used for main propulsion. Its purpose is to automatically release excess steam pressure to prevent damage to the turbine in the event of a malfunction or overpressure situation. This is a required safety feature on steam turbine-powered vessels according to Coast Guard regulations. The other options are incorrect because they do not specifically pertain to the steam turbine casing. An HP turbine bypass valve, sliding beam, and duplex relief valves are not required components on the steam turbine itself, though they may be found elsewhere in the propulsion system.