Where should pipe joint compound be applied?

The correct option emphasizes the application of pipe joint compound to male threads, specifically advising to leave the last two threads free of compound. This practice is essential for ensuring a proper seal and preventing contamination of the internal fluid flow. By omitting the last two threads, you reduce the risk of material entering the pipe, which can cause blockages or interfere with the flow characteristics of the liquid or gas being transported.

Applying the compound strictly to the male threads enhances the connection while ensuring that it does not interfere with the female threads' ability to engage properly. This helps maintain a solid mechanical connection and allows for any excess compound to escape without compromising the integrity of the joint.

Using pipe joint compound on both male and female threads (which might be tempting for added sealing) could lead to over-application, causing a messy assembly and potential blockages. Therefore, the correct approach is to focus on just the male threads up to the last two, preserving the integrity of the threading and the overall function of the pipe system.