Which of the following is an allowed taping of a water column?

A steam pressure gauge is an allowed taping of a water column because it provides critical information about the pressure within the boiler system. The water column is essential for accurately measuring the water level in a boiler, and having a connection for a steam pressure gauge ensures that operators can monitor the steam pressure directly related to the boiler's performance.

This taping helps in maintaining safe operation by allowing for real-time monitoring of pressure that can influence the boiler's water level and overall functionality. It is crucial for maintaining the proper functioning and safety of the boiler, as the pressure and water level are directly correlated in steam systems.

In contrast, a thermostatic valve, condensate return line, and gas regulator do not serve the primary function of measuring or indicating water levels within a boiler's water column. Each of these components serves different purposes in the operation of a boiler system, such as regulating temperature, returning condensate, or controlling gas flow, none of which are directly related to the necessary measurements provided by a water column.