Couplings and unions for standpipe systems must be of a type specifically listed for what purpose?

The correct answer focuses on the requirement that couplings and unions used in standpipe systems must be specifically listed for use in those systems. This is crucial because standpipe systems are a key component of fire protection and must comply with certain safety regulations and performance standards.

Using parts that are specifically listed ensures that they have been tested and certified for their intended application, which includes characteristics like pressure tolerance, durability, and compatibility with fire suppression systems. Standpipe systems are designed to deliver water to combat fires, and any components used in these systems must operate effectively under high-stress conditions to ensure reliable performance during emergencies.

In contrast, the other options address purposes that while important in their own right, do not capture the specific regulatory and safety requirements for components in standpipe systems. Thus, they do not represent the main criteria needed for couplings and unions in this context.