A riser valve should be adjacent to and labeled as:

A riser valve is a crucial component in any medical gas system as it controls the flow of gases throughout the facility. Being adjacent to mainlines ensures that it can be easily accessed and serviced when needed, without disrupting the overall gas delivery system. Additionally, proper labeling as "Mainline" provides clarity and helps staff identify the purpose of the valve quickly, facilitating efficient and safe interactions during routine maintenance or emergency situations.

Accessibility is essential because medical gas systems must be operational without delay, particularly in emergency or critical care scenarios where time is of the essence. By ensuring that these valves are not only accessible but also clearly marked, facilities can comply with safety regulations and reduce the risk of confusion during high-pressure moments.

In contrast, the other options, while they mention aspects of safety or regulatory compliance, do not emphasize the critical need for ease of access and proper labeling that are fundamental to the function and safety of medical gas systems. For example, labeling something as "pressure-tested" does not inform personnel on its operational status or its location relative to critical areas. Thus, the answer highlights the best practices in valve installation and maintenance that align with both safety protocols and operational efficiency.