Which of the following fittings is not allowed to be concealed according to gas fitting regulations?

The selection of "concealed gas fittings" as the correct answer is based on safety regulations and standards that govern gas installations. Concealed fittings, by their nature, are hidden from view, making it difficult to inspect and maintain them. Allowing concealed gas fittings can introduce risks such as undetected leaks, which can lead to hazardous situations, including explosions or carbon monoxide exposure. Thus, regulations typically require that certain fittings must remain accessible for safety reasons.

In contrast, unions, compression fittings, and right and left couplings are types of fittings often allowed in concealed spaces because they are designed for specific functions within the plumbing and gas systems and generally can be accessed for maintenance without needing to dismantle extensive sections of the installation. These fittings are not inherently dangerous when concealed as long as their installation meets safety codes and standards.