Which of the following pertains to the modification of fitting sizes in an irrigation setup?

The modification of fitting sizes in an irrigation setup is best addressed by using a bushing. A bushing is a type of fitting that allows for the connection of two pipes or fittings of different sizes, enabling a smooth transition from one diameter to another. This is particularly important in irrigation systems where different components may have varying sizes due to system design or the need to accommodate different flow rates. By using a bushing, technicians can ensure that connections are secure and prevent leaks, while also allowing for efficient water flow through the system.

In contrast, control wires are primarily associated with the operation of electrical components, check valves are used to prevent backflow and ensure proper water flow direction, and booster pumps serve the function of increasing water pressure within the system. None of these components directly address the need for modifying pipe sizes, highlighting why a bushing is the correct answer in this context.