Which agency's guidelines dictate the markings required when storing LP-Gas and other gases together?

The correct choice is the National Fire Protection Association (NFPA) because this organization provides comprehensive guidelines and standards specifically related to fire safety, including those for the storage and use of LP-Gas and other flammable or hazardous gases. The NFPA codes establish the necessary markings and labeling to ensure proper identification and management of gas storage systems, focusing on safety measures to prevent accidents and enhance emergency response.

In the context of LP-Gas storage, NFPA guidelines help ensure that all necessary precautions are taken to mitigate risk and provide clear communication about the potential hazards associated with different gases. This includes recommendations for signage, labeling, and other important markings that inform those working in or around these environments about safety requirements and protocols.

The other agencies listed, while involved in safety and environmental protection in different capacities, do not specifically address the marking requirements for LP-Gas storage in the same direct manner as the NFPA. The U.S. Environmental Protection Agency focuses more on environmental regulations, the Occupational Safety and Health Administration deals with workplace health and safety regulations broadly, and the Department of Energy addresses energy policies and standards rather than specific guidelines on marking for gases.