Which of the following plumbing fixtures requires the least water flow capacity?

The sink requires the least water flow capacity among common plumbing fixtures. When considering the flow rates, sinks typically operate effectively with a flow rate of around 1.5 to 2.5 gallons per minute (GPM), depending on the faucet design and purpose. This lower requirement is mainly because sinks are generally used for tasks that do not demand a high volume of water, such as washing hands or rinsing small items.

In contrast, water closets often need a higher flow capacity to efficiently flush waste, with standard fixtures using anywhere from 1.6 to 3.5 gallons per flush. Showers generally require a greater flow rate as well, often around 2.5 GPM for optimal performance. Bathtubs require a significant amount of water to fill, typically needing several gallons, which far exceeds the requirements of a sink. Therefore, in terms of water flow capacity, the sink is the least demanding plumbing fixture.