What safety feature is required for fill openings?

The requirement for a locking closure device at fill openings is essential for ensuring safety during the refueling process. This feature minimizes the risk of accidental spillage or unauthorized access, which could lead to hazardous situations. By securing the fill openings with a locking mechanism, the integrity of the system is maintained, preventing possible leaks that could pose environmental or safety hazards. This is especially important in various applications, including fuel tanks and other liquid storage systems, where controlling access and preventing spills is critical for safety and compliance with regulations.

The other choices represent important safety measures in various contexts but do not specifically address the need to secure fill openings against accidental or unauthorized operation during filling processes.