Which type of connection is used to make a single change in the direction of piping at a 90° angle?

The correct choice for making a single change in the direction of piping at a 90° angle is the 90° elbow. This fitting is specifically designed to facilitate this type of directional change in piping systems. The 90° elbow creates a sharp turn that allows fluid to flow around corners and navigate through different layouts in plumbing and HVAC systems, ensuring efficient routing without causing excessive pressure loss or flow disturbances.

In contrast, a reducer serves to connect pipes of different diameters, a coupling is used to connect two pieces of pipe in line, maintaining a more linear flow, and a return bend is utilized in specific applications to change direction more smoothly over a 180° turn. Each of these alternatives serves distinct purposes in piping systems, further emphasizing the unique role of the 90° elbow for achieving a sharp directional change.