Which of the following is NOT recognized as a standard fitting?

A 45 degree tee is not recognized as a standard fitting. Standard fittings typically include components that serve specific, commonly encountered purposes in plumbing systems. The 90 degree elbow is designed to change the direction of piping by 90 degrees, while a coupling connects two pieces of pipe in a straight line. Similarly, a cap is used to seal the end of a pipe, preventing flow.

In contrast, a 45 degree tee is less common and does not have the same standardized application as the standard fittings listed above. Registries or plumbing codes may not necessarily recognize a fitting that serves such a niche function in comparison to the more fundamental roles of elbows, couplings, and caps. Thus, the lack of prevalence and standardization for the 45 degree tee differentiates it from the other options, making it the correct answer in this context.