Nonmetallic boxes must be suitable for the lowest ___ conductor entering the box.

The correct choice is the one that refers to temperature ratings because nonmetallic boxes are primarily concerned with their ability to withstand heat generated by electrical conductors entering the box. When considering the safety and durability of the materials used in nonmetallic boxes, the lowest temperature-rated conductor ensures that the box can handle the maximum operating temperature without degrading or posing fire hazards.

Nonmetallic boxes have specific temperature ratings based on the materials they are made from. If a conductor with a higher temperature rating is used, it can operate safely without risk. However, if the conductor has a lower rating than the box, it can lead to overheating and potential failure. Thus, it's essential to ensure that the box is rated appropriately for the conductors it will house.

While the other options—current-rated, voltage-rated, and resistance-rated—are significant in electrical installations, they do not pertain specifically to the safety and installation requirements of nonmetallic boxes in relation to temperature.