What is the tank called where treated water is stored at the plant?

The correct answer is the clear well. A clear well is specifically designed for the storage of treated water at a water treatment plant. Its primary function is to provide a volume of water that ensures that the distribution system can be efficiently and reliably supplied with potable water while maintaining chlorine residuals and preventing contamination.

Holding tanks, while they can store water, are typically used for intermediate storage in various stages of water treatment or for temporary storage. Reservoirs are larger and often provide storage for water supply at the community level, not specifically at the treatment plant. Distribution tanks are part of the distribution system and store water that has been treated and is ready to be distributed to consumers, but they are not usually located within the treatment plant itself. In summary, a clear well is specifically engineered for the purpose of storing treated water immediately after treatment, making it the most appropriate term for such a facility at a treatment plant.