According to the NEC, which statement is true regarding receptacles?

The correct answer focuses on the requirement that receptacles must be marked with the manufacturer's name or identification. This is essential for ensuring traceability and accountability in electrical systems. When receptacles are properly marked, it allows for easy identification of the manufacturer, which can be critical in the event that there are safety issues, recalls, or warranty claims related to the product. This requirement is part of the broader safety and quality control measures outlined in the National Electrical Code (NEC), which establishes a standard to promote uniformity and safety in electrical installations.

The other options do not accurately reflect the NEC requirements for receptacles. While cover requirements for unused receptacles may be relevant in certain contexts, marking with the manufacturer's identification is a fundamental and specific regulatory requirement that reinforces safety and accountability. Additionally, while some testing standards may apply to other electrical components, typical receptacles do not have an annual testing requirement stated within the NEC. Lastly, while receptacles can vary in design to some extent, there are specific guidelines that govern their form and function, and these guidelines are not entirely without restrictions.