Which part of the NEC® states that unused knockouts in electrical boxes must be closed?

The requirement for unused knockouts in electrical boxes to be closed is specifically addressed in Article 300 of the National Electrical Code (NEC). Article 300 outlines general requirements for wiring methods and materials, including the necessity for electrical boxes to prevent hazards such as exposure to tampering, moisture, or the ingress of foreign objects, which can occur if knockouts are left open. Closing these knockouts ensures that the internal components of the box remain safe and that the integrity of the entire electrical installation is maintained.

Even though there are other articles that discuss various components of electrical installations, the specific regulation regarding closing unused knockouts is found in Article 300. This reinforces the importance of following the NEC guidelines to ensure safety and compliance in electrical work. The inclusion of all other articles in the answer option does not apply since they do not specifically address the closing of knockouts.