Which component of a water closet flush tank is responsible for opening the flush?

The component responsible for opening the flush in a water closet flush tank is the discharge valve. When the flush handle is activated, it lifts the flapper or opens the valve, allowing water from the tank to flow into the bowl, thereby flushing the toilet. The discharge valve is crucial because it controls the release of water, achieving the primary function of the flush mechanism.

While the fill valve is responsible for refilling the tank after a flush, and the overflow pipe is a safety feature to prevent overflow by directing excess water away from the tank, their roles do not involve initiating the flush process. The flush handle serves as the manual trigger for flushing but does not directly open the valve itself; rather, it operates the discharge valve. Hence, the discharge valve is the correct choice as it is the component that directly facilitates the flushing action.