The equipment supplying the circuits must be marked where plainly visible to indicate each circuit that is Class 2 or Class 3. What is the appropriate term for this marking?

The appropriate term for the marking of equipment supplying circuits designated as Class 2 or Class 3 is "durably." This means that the markings must be made with materials or methods that ensure they will withstand environmental conditions and the wear and tear that can occur over time.

Durability is critical because it guarantees that the markings remain legible and clearly identify the classification of the circuits throughout their service life. This is essential for safety and compliance with code requirements since it helps ensure that anyone working on or inspecting the equipment can readily identify the circuit class. If the marking were to fade or become illegible, it could lead to unsafe practices or misunderstandings regarding the nature of the electrical circuits.

The other terms provided do not accurately reflect the permanence necessary for such markings. For example, "temporarily" would imply a short-lived marking, which would not meet the required visibility and longevity standards. "Occasionally" lacks the meaning of permanence or consistency required for safety specifications. Hence, "durably" is the only suitable option for this context.