Final answer:
The correct pipe fitting for 90-degree connections of different-sized pipes is a reducer tee, allowing for size transition and directional change.
Step-by-step explanation:
Within the context of pipe fittings and plumbing, the correct answer for making different-sized branch line connections at 90° is d. Reducer tees. A reducer tee is specifically designed to connect three sections of pipes where the main line and the branch are of different sizes, which allows for a change in pipe diameter while also changing the direction of flow at a 90-degree angle. This contrasts with other fittings such as nipples, which are short sections of pipe typically used to connect two other fittings; elbows, which change the direction of flow without changing the pipe size; and return bends, which are U-shaped fittings that create a 180-degree bend in the piping.
For example, connecting a 4-inch pipe to a 16-inch pipe in a chemical plant might be necessary for efficient processing and flow management. This scenario illustrates the concept of economies of scale in the chemical industry, where larger pipes such as the 16-inch pipe might be used for the main flow, and smaller pipes like the 4-inch pipe serve as branch lines for different processing areas.