Where would you typically find a flush valve?

The flush valve is a key component of a toilet, specifically located at the base of the tank. Its primary function is to release water from the tank into the toilet bowl during a flush, allowing for proper waste removal. This mechanism is integral to the flushing action and helps ensure that the toilet operates effectively.

In contrast, the other options do not correctly describe the function or location of a flush valve. In sinks, you would find faucets and drain systems, but not a flush valve. Water heaters contain various valves and components, but they are not involved in the flushing process characteristic of toilets. Similarly, while a shower system has valves for controlling water flow, it does not involve a flush valve; instead, it uses diverters, mixing valves, or shower heads. Thus, the flush valve is specifically associated with toilet mechanisms, making the identification of its location at the base of a tank for toilets the correct answer.