Final answer:
Type K copper tubing does not require a protective coating when installed underground due to its thick wall, making it suitable for corrosive environments.
Step-by-step explanation:
When installed underground, Type K copper tubing does not require a protective coating. There are various types of copper tubing, each with different thicknesses and specific uses. Type K has the thickest wall and is considered suitable for use underground where the conditions may be more corrosive. It is more durable and has a longer lifespan in these environments compared to Type L, Type M, or Type DWV copper tubing. Type K's durability is due to its thick wall, which provides additional resistance to external corrosion without the need for supplementary protection.