Which of the following is an example of an ordinary domestic water tap?

The correct response identifies a faucet as an ordinary domestic water tap. A faucet is a common fixture found in homes that regulates the flow of water. It typically consists of a spout and a handle, allowing users to turn the water on and off and often to adjust the temperature. This makes it the standard component in kitchens, bathrooms, and other areas where water is needed for daily activities.

Other options relate to components that serve different functions in plumbing systems. While a stem is part of the mechanism that operates a faucet, it does not refer to the entire assembly that dispenses water. A valve seat is another component within a tap assembly that helps seal the water flow when it is turned off, but it too is not a standalone tap. A plug usually refers to a device that blocks openings or drains but does not regulate or dispense water like a faucet does. Thus, recognizing that a faucet is the complete fixture designed for dispensing water clarifies its role as the correct answer.