What tool was used by the instructor to check the safety of the panel cover and screws?

The correct answer pertains to the tool used to ensure the safety of the panel cover and screws. The instructor likely used a "ticker," which is commonly associated with checking for live electrical current or detecting voltage in electrical panels. It serves the purpose of ensuring that the panel is safe to work on or inspect by confirming whether or not it is energized.

This tool is vital because if a panel is live, using inappropriate methods to open it or make adjustments can lead to electric shock or accidents. While a gauge could measure dimensions or other properties, and a screwdriver would primarily be for securing or removing screws, neither addresses the safety aspect associated with checking for live circuits. A tester, depending on its type, may measure voltage or continuity, but the specific function of a "ticker" is more specialized for detecting live circuits in panels, making it the most appropriate tool for the task described.