The safety valve nominal size for propulsion boilers and superheaters must be not less than 1 1/2 inches and not more than 4 inches. The term 'nominal size' refers to the __________.

Correct Answer: Option B — diameter of the inlet opening

In accordance with 46 CFR marine engineering standards, the nominal size designation for propulsion boiler and superheater safety valves specifically refers to the internal diameter of the valve inlet opening where steam enters from the pressure vessel.