How must drain lines be installed with respect to other utilities?

Drain lines are required to be installed below water supply lines to ensure that any potential leaks in the water supply do not contaminate the drain system. When drain lines are placed above water supply lines, there is a risk that any drainage leaks could flow down to the water supply lines. This could lead to serious health hazards due to the possibility of cross-contamination.

Keeping drain lines below water supply lines follows principles of safety and plumbing code regulations, which are aimed at preventing the backflow of waste into clean water systems. This is particularly important where potable water is involved, as maintaining a clear and safe separation helps ensure the integrity of the entire plumbing system.

The other options do not create a safe separation, hence they do not adhere to established plumbing guidelines. Therefore, the installation of drain lines below water supply lines is the appropriate and safest practice in plumbing systems.