What type of plumbing fixture is designed to mix both hot and cold water?

The correct answer is that a faucet is specifically designed to mix both hot and cold water. Faucets include components that allow users to control the temperature of the water by adjusting the flow of hot and cold lines. This mixing is usually facilitated by a single handle or two separate handles, depending on the design of the faucet.

While lavatories, sinks, and showerheads can incorporate faucets and may include mixing options as part of their configuration, they are broader categories of plumbing fixtures. A lavatory refers to a basin typically found in bathrooms, which may have a faucet installed but does not specifically denote the mixing capability. Similarly, a sink is a general term that can apply to various types of basins used in both kitchens and bathrooms, and while many sinks feature faucets for mixing water, they are not defined by that feature alone. A showerhead delivers water but does not mix it; instead, it relies on a separate control or valve for temperature regulation.

Therefore, the faucet is the most accurate choice as it directly refers to the mechanism that enables the mixing of hot and cold water for various fixtures.