Final answer:
A safety guard, such as guard railings or a cover, is required for floor openings measuring at least 12 inches in the least dimension to ensure safety and prevent falls.
Step-by-step explanation:
The protective device required for a floor opening that measures at least 12 inches in its least dimension is typically a safety guard, such as a guard railing or a cover. In occupational settings, such as construction sites, OSHA (Occupational Safety and Health Administration) standards dictate that floor openings must be protected to prevent workers from falling through. These protective measures are crucial for preventing accidents and ensuring the safety of everyone in the area.