Which of the following methods is used in plumbing systems installation?

The method used in plumbing systems installation that is most pertinent here is back-to-back installation. This approach involves placing two or more fixtures directly adjacent to each other, allowing for a more efficient plumbing layout. By organizing fixtures in this manner, it simplifies the connection to the main plumbing lines, often reducing the amount of piping required and minimizing installation time and costs.

Back-to-back installation is particularly advantageous in multi-unit residential settings where multiple bathrooms or kitchens share wall space. This configuration facilitates easier access for maintenance and repairs while optimizing the use of available space within the structure.

The other choices do not provide the same level of efficiency or practicality in plumbing systems. Single-outlet installations may only serve one fixture, which can limit the overall design. Linear installation is more common in ductwork systems rather than plumbing. Open duct installations pertain more to HVAC systems and do not relate directly to plumbing installations.