An example of a bypass in a domestic water system is...

A pipe extending from before to after a valve is an example of a bypass in a domestic water system because it allows water to flow around the valve without going through it. This configuration can be particularly useful for maintenance purposes; when the valve needs to be shut off for repairs or adjustments, the bypass enables the system to continue to function without interruption. By diverting the water flow, the system can maintain adequate pressure and supply to downstream fixtures or areas while the valve is out of service.

In contrast, the other options refer to different aspects of water distribution or redundancy in a plumbing system rather than a direct bypass mechanism. For instance, a redundant water line is typically installed for emergency situations but does not serve the immediate function of allowing water to bypass a valve. A temporary water service connection is designed for short-term use and is not a permanent feature of the system. Similarly, a secondary water heater is an added component for providing hot water but does not facilitate bypassing other parts of the system.