Final answer:
When a tire has a nail hole in its sidewall, the proper action is to replace the tire, as repairs to the sidewall are generally not recommended and can compromise the tire's safety.
Step-by-step explanation:
If a tire is found to have a nail hole in its sidewall, the correct course of action would be D. Replace the tire. Repairing a tire with a patch from the inside or with a string type plug is generally only suitable for damage that occurs on the tread area of the tire.
The sidewall of a tire is responsible for sustaining the tire's structure and is under constant flexing, which makes repairs in this area not reliable for the long term. Plus, most manufacturers do not recommend repairing the sidewall of a tire as it can compromise the tire's structural integrity. Therefore, sidewall damage typically necessitates tire replacement to ensure safe driving conditions.