Which article pertains to wiring systems that are installed beneath flooring?

The article that pertains to wiring systems installed beneath flooring is indeed Underfloor Raceways. This article provides guidelines for the design, installation, and use of raceways specifically intended for accommodating electrical conductors and wiring in an underfloor environment.

Underfloor raceways are particularly significant in modern buildings where access to electrical wiring needs to be both secure and flexible for future modifications. These raceways help to organize and protect electrical systems within the flooring structure, allowing for easy access while maintaining the integrity and safety of the installation.

The other options do not address underfloor installations specifically. Surface Nonmetallic Raceways focus on raceways installed on surfaces rather than below them. Cable Trays are designed for supporting cables and conductors but are typically mounted overhead or on walls, not under floors. Open Wiring on Insulators refers to an older, less common method of wiring that doesn't apply to modern underfloor raceway systems. Therefore, Underfloor Raceways is the most relevant article for this question.