How frequently must a person observe both the cargo tank and the receiving container when unloading a cargo tank with a capacity of 3500 water gallons or less, if the metered delivery service takes more than five minutes?

The correct choice is based on the specific requirements outlined in NFPA 58 regarding the safety protocols for liquid propane gas (LPG) transfers. When unloading a cargo tank with a capacity of 3500 water gallons or less, the regulation stipulates that the person responsible for overseeing the delivery must observe both the cargo tank and the receiving container at least once every 5 minutes when the metered delivery service exceeds five minutes. This observation is crucial to ensure that any issues, such as leaks or overfill situations, can be promptly detected and addressed to minimize risks associated with LPG handling. Regular monitoring helps maintain safety during the transfer process, adhering to best practices for managing hazardous materials.

Other choices propose observation frequencies that are either more frequent than necessary, which could lead to unnecessary distractions from the task at hand, or less frequent than what is mandated by regulation, thereby compromising safety. This requirement reflects the balance between vigilance in monitoring and practical operational efficiency during liquid transfers.