What feature allows the user to flush a water closet with variable water volume?

The correct choice is a dual flushing device, which is designed to offer two different flush volumes for a water closet. This feature allows users to select between a full flush for solid waste and a reduced flush for liquid waste, effectively conserving water.

By giving users the option to use a lower volume of water when appropriate, dual flush toilets align with modern water conservation efforts and regulations. This functionality is particularly beneficial in reducing water consumption in residential and public restrooms, making it an environmentally friendly choice.

While other flushing devices may be designed for specific functions or to provide different types of pressure, they do not offer the same flexibility in adjusting the water volume based on the waste type. For instance, single flushing devices typically use the same amount of water for every flush, which may not be as efficient in terms of water usage compared to a dual flushing system.