What is one requirement for containers used in mobile service?

For containers used in mobile service, one crucial requirement is that they must always be securely fastened. This ensures safety during transport, preventing the containers from tipping over or becoming dislodged, which could lead to leaks or accidents involving liquid propane gas. Proper fastening also minimizes the risk of containers rolling around, which could cause damage to the containers and the vehicle, as well as pose hazards to personnel and the public.

The other options may represent considerations in certain contexts, but they do not meet the critical safety requirement established for mobile service. Bright colors may enhance visibility, but they do not ensure safety. Wheels are not a necessity for all types of containers used in mobile service. Similarly, while it is often beneficial for containers to be empty before transport, it is not a universal requirement and can depend on specific circumstances and regulations concerning the nature of the propane being transported.