Explanation:
The rubber device that seats on the flush valve when the flush is completed is called the flapper. The flapper is a crucial component of the water closet flush tank, as it forms a watertight seal between the flush valve and the tank when the toilet is not in use. When the flush is initiated, the flapper lifts, allowing water to flow from the tank into the bowl, and then reseats itself on the flush valve once the flush is completed, preventing water from continuously flowing into the bowl. This mechanism helps maintain the water level in the tank and ensures efficient flushing while preventing water wastage.