What type of compound must be resistant to the action of liquefied petroleum gas?

The correct choice emphasizes the importance of using a thread compound that is specifically resistant to liquefied petroleum gas (LPG). Thread compounds play a crucial role in ensuring leak-proof connections in gas piping systems. These compounds create a seal on the threaded joints, preventing any gas from escaping which can pose a significant safety hazard.

In the context of propane and other types of LPG, using a thread compound that can withstand chemical degradation is essential, as LPG can break down certain materials or lead them to lose their sealing properties over time. A properly formulated thread compound enhances the integrity of the gas system and helps maintain safety standards.

Other substances like sealants or caulks may not be designed for the specific conditions presented by LPG. While they may work in different applications, they could fail to maintain a reliable seal under the pressures and chemical characteristics of propane. Furthermore, grease is typically unsuitable for gas connections, as it can mix with the gas and create more opportunities for leaks or unsafe conditions. Thus, a resistant thread compound is the best practice in installations involving liquefied petroleum gas.