Which color insulation shall not be used for ungrounded or grounded conductors?

For ungrounded or grounded conductors, green insulation should not be used because it is specifically designated for grounding purposes only. Grounding conductors are meant to provide a path to ground to prevent electric shock and ensure safety, hence they must be distinctly identifiable to avoid confusion with conductors that carry current.

While red and black insulation are both permissible for ungrounded (hot) conductors, the inclusion of green in the options emphasizes the unique safety role it plays. Therefore, recognizing that green insulation is reserved for grounding highlights the importance of following color-coding standards in electrical work to maintain safety and compliance with regulations.

Thus, the correct understanding leads to the conclusion that only the use of green insulation should be avoided for ungrounded or grounded conductors, affirming your choice.