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

The dual flushing device is designed to give users the option of flushing with two different volumes of water. This mechanism typically includes two buttons or levers – one for a small flush for liquid waste, and another for a larger flush intended for solid waste. This feature not only allows for more efficient water use, accommodating different flushing needs, but it also promotes water conservation by enabling the user to choose a reduced volume of water when a full flush is not necessary.

This contrasts with other types of flushing mechanisms. The single flushing device operates with a fixed volume of water, which does not allow for any variation based on the type of waste being flushed. The variable pressure device typically adjusts the pressure of the water to improve efficiency but does not specifically offer variable volumes based on user choice. An eco-flush mechanism, while often aimed at conserving water, is generally not defined as having dual volume options but rather could signify a system designed to use less water overall without the user actively selecting the flush volume. Thus, the dual flushing device is the most suitable choice for providing variable water volume for flushing a water closet.