Which of the following is NOT a main component of a water supply system?

The correct choice is D, the water storage tank is not commonly categorized as a main component of the water supply system in the same way that the other components are. In a typical water supply system, the key elements include:

1. The source, which can be a surface water body like a river or lake, or groundwater from aquifers, serves as the starting point from which water is extracted.

2. The treatment process, which involves various methods such as filtration, chlorination, and sometimes fluoridation, is essential to ensure that the water meets health and safety standards before being supplied to consumers.

3. The distribution network consists of the pipelines, pumps, and valves that transport treated water from the treatment facility to the end users. This system is crucial for delivering water efficiently and effectively.

While water storage tanks do play an important role in a water supply system by holding and stabilizing water supply, they are often considered more as ancillary components that support the main system rather than core components. Their primary function is to ensure a reliable supply during peak usage times or emergencies, rather than being a direct part of the sourcing, treatment, or distribution of water.