Which of the following must not be used in the drainage system?

The option that must not be used in the drainage system is running threads and bands. This is because running threads and bands can compromise the integrity of the joints, leading to potential leaks and structural failures in the drainage system. The International Residential Code emphasizes the need for reliable connections in plumbing systems to prevent water loss and contamination, ensuring that all joints are secure and capable of handling the pressures and conditions of waste disposal.

In contrast, plastic fittings and cast iron are specifically designed for durability and resistance to corrosion. They are commonly used materials in plumbing systems, providing strength and longevity under various conditions. Compression joints are also typically acceptable because they create a tight seal without the need for additional threading, which helps maintain the integrity of the drainage system.

In summary, the prohibition against using running threads and bands in drainage systems is grounded in the need for secure, reliable connections that prevent leaks and maintain system performance.