What type of valve must be held open manually by the operator when filling one-pound cylinders?

The correct choice is the snap-acting "dead man" valve. This type of valve is designed to require manual activation by the operator when filling one-pound cylinders to ensure safety during the dispensing process. The manual operation means that the valve will only remain open while the operator is actively holding it, thereby preventing unintentional dispensing of LP-gas if the operator were to let go or become distracted.

This safety feature is crucial because it minimizes the risk of overfilling or accidental release of gas, which could lead to hazardous situations. The design of this valve is specifically intended to promote safety by ensuring that the operator maintains control over the gas flow throughout the filling process.