___________ shall NOT be used for upper and lower casings of regulators.

The appropriate answer highlights the safety and functional requirements for regulator casings. Non-metallic parts should not be used for the upper and lower casings of regulators because they typically lack the necessary durability and resistance to high pressure and temperature fluctuations that are critical in propane systems. Regulators operate under conditions that can stress materials, and non-metallic materials may not withstand such conditions reliably over time, posing a risk of failure.

In contrast, metallic parts, plastic parts, and composite parts can potentially provide suitable strength and resilience for the task, depending on their specific design and material properties. Thus, the choice of using non-metallic parts is not recommended for these applications due to concerns over integrity and safety in propane delivery systems.