Final answer:
The residual water pressure should be at least 20 psi (option B).
Step-by-step explanation:
The residual water pressure after allowing for friction and other pressure losses should be at least 20 psi (option B).
Pressure losses in a water system can occur due to factors such as frictional resistance in pipes, bends, valves, and other components. To ensure a sufficient residual water pressure, a minimum value needs to be maintained. In this case, it should be at least 20 psi.
This minimum pressure requirement is important for proper functioning of appliances and fixtures that rely on water pressure, such as showers, faucets, and toilets.