For horizontal runs directly under a floor drain, which should be connected?

The connection that should be applied for horizontal runs directly under a floor drain is the P-trap. A P-trap is essential in plumbing systems, specifically for floor drains, because it serves multiple critical functions. Primarily, it creates a water seal that prevents sewer gases from entering the building through the drain. This seal is maintained by the water that remains in the trap, which blocks any odor while allowing wastewater to flow through.

In horizontal runs, using a P-trap ensures that any wastewater efficiently drains away while maintaining the necessary barrier against unpleasant and potentially hazardous fumes. This is particularly important in areas where drains are close to living or working spaces.

The other options do not provide the essential function of maintaining a water seal. A sleeve typically refers to a protective covering, an elbow is used to change the direction of a pipe, and a nipple serves to connect two fittings but does not have the necessary features to function as a trap. Thus, the appropriate choice for connecting to a floor drain in this scenario is indeed the P-trap.