The only type of fixture trap permitted is a(n) ___-trap.

The P-trap is the only type of fixture trap permitted due to its efficacy in preventing sewer gases from entering living spaces while also serving to catch debris that might otherwise clog the plumbing system. The design of a P-trap includes a curved shape that retains a small amount of water, which acts as a barrier to odors and gases.

In addition to its functionality, the P-trap is relatively straightforward to clean and maintain, making it a practical choice for plumbing systems. Its design allows for easy inspection and clearing of clogs, which is essential for effective plumbing management in residential and commercial buildings.

The other types of traps mentioned—such as the S-trap, which is not allowed because it can siphon and allow gases to escape, or the Q-trap and C-trap, which are not standard or commonly recognized designs—do not meet the necessary standards for performance and safety in plumbing systems. Therefore, the P-trap is recognized as the most effective and compliant option.