Which fittings are categorized as "concentric" couplings?

Concentric couplings are fittings used to connect two pipes of the same diameter, where the centerlines of both pipes are aligned. This type of coupling facilitates a smooth transition or connection without any offsets in the pipe alignment, which is critical for maintaining fluid flow and reducing turbulence.

The correct selection identifies fittings that share this characteristic of concentric alignment. When reviewing the fittings labeled I, III, and VI, each fitting must allow pipes to be connected in a manner where their centers align directly with one another. This feature supports effective plumbing practices, ensuring a stable and leak-free connection.

In contrast, other combinations of fittings may include eccentric couplings or fittings designed for specific purposes that do not maintain concentricity, such as those that accommodate changes in pipe diameter or layout that require an offset. Thus, focusing on the correct answer highlights the importance of understanding fitting types and their appropriate applications in plumbing systems.