Which fixture's final waste connection cannot be made using a compression gasket?

The reason why the final waste connection for a bathtub cannot be made using a compression gasket is primarily due to the nature of the waste connection and the requirements for effective sealing. Bathtubs often have larger drain openings and require a more secure and permanent connection to prevent leaks and ensure proper drainage.

In typical plumbing practice, bathtubs are often connected using solvent welds, threaded connections, or other methods that provide a robust seal. These methods are essential because the bathtub must handle significant water flow and potential weight from bathers, making a secure connection critical to prevent leaks.

On the other hand, sinks, toilets, and showers can accommodate compression gaskets for their waste connections, as these fixtures do not experience the same level of stress and flow conditions that bathtubs do. Compression gaskets are often used in areas where access may be required for maintenance and where a less permanent connection is acceptable. However, for bathtubs, the choice to avoid compression gaskets in favor of a more durable connection underscores the importance of ensuring the reliability of the waste system in this context.