Why is a drain cock or drain valve required in a water column?

The correct reasoning for why a drain cock or drain valve is required in a water column centers around its function in maintenance and operational efficiency. A drain valve allows for the removal of water from the system, which is essential for draining and purging any sediment that may accumulate, thereby maintaining the accuracy of the water column. It facilitates maintenance by allowing the system to be checked and serviced without major disassembly, ensuring the proper functioning of the water column.

While other options present relevant concepts, they do not directly address the primary purpose of a drain cock or valve in this context. For example, maintaining pressure is important but is generally managed through other system components. Preventing backflow is crucial in certain systems but does not specifically relate to the functionality of the drain valve in a water column. Lastly, controlling temperature pertains to thermodynamic processes rather than the draining capability of the water column.