Which plumbing fitting is used to change the direction of flow by 90 degrees?

The correct choice for changing the direction of flow by 90 degrees in plumbing is the elbow fitting. An elbow is specifically designed to facilitate a directional change in piping systems, allowing the flow to pivot either left or right, or up or down, depending on its configuration. This makes it an essential component when routing plumbing in spaces where straight runs aren't feasible.

Other fittings serve different purposes. A connector is used to join two lengths of pipe but does not alter the flow direction. A reducer is implemented to connect pipes of different diameters, which aids in transitioning the flow but does not change its direction. A coupling is meant for connecting two sections of pipe in a straight line, without any angle alteration. Thus, the elbow fitting is the optimal choice when a 90-degree direction change is required.