Which of the following materials is NOT permissible for use as underlayment for hardwood flooring?

The correct choice highlights particle board as an unsuitable underlayment material for hardwood flooring due to its properties and moisture susceptibility. Particle board is made from wood particles bonded together with adhesives under heat and pressure, and it can absorb moisture easily. This absorption can lead to swelling, warping, and overall deterioration, compromising the integrity of the hardwood flooring laid over it.

In contrast, plywood serves well as an underlayment because it provides a solid and stable base, and it has good moisture resistance when properly treated. Felt paper is often used as a moisture barrier and can help reduce noise, making it a suitable option. Cement board is also acceptable due to its moisture resistance and durability, especially in areas where water exposure may be a concern. These materials enhance the performance and longevity of hardwood flooring installations, making particle board the clear outlier in this context.