Which of the following is permitted on substrates beneath SFRM?

The correct choice is bonding agent because it serves a vital function in ensuring the spray-applied fire-resistive materials (SFRM) adhere effectively to the substrate. A bonding agent enhances adhesion on surfaces that may otherwise hinder the performance of the fireproofing material. This is particularly relevant when dealing with non-porous or smooth substrates, where achieving a solid bond is crucial for the integrity and effectiveness of the fire protection system.

In contrast, options like cement board and wood planks, while they may be suitable substrates for certain applications, are typically not required or necessary in conjunction with SFRM, as SFRM is designed to be applied directly to structural elements. Plastic sheeting could prevent proper bonding or may not provide a suitable surface for SFRM application, as it is often not a conducive substrate for fireproofing materials due to its non-porous nature and potential for slippage.