Which of the following fittings would be described as reducing fitting?

A reducing fitting is specifically designed to connect pipes or tubes of different diameters. This allows for a transition from a larger diameter pipe to a smaller one or vice versa. In plumbing, these fittings are essential for systems where a reduction in pipe size is necessary, such as when connecting a main supply line to a branch line.

The significance of the reduced end size is important for maintaining proper flow rates and pressures in the plumbing system. When a fitting has different end sizes, it effectively facilitates this size transition, which is the defining characteristic of a reducing fitting.

Other options do not accurately describe a reducing fitting. For instance, a fitting with the same outer diameter does not change the size of the pipe connected and therefore does not qualify as a reducing fitting. Similarly, fittings used for gas lines or that merely connect two pipes do not inherently imply a size reduction unless specified. Thus, the correct answer highlights the primary function and design of a reducing fitting in plumbing applications.