The net cross-sectional area on any plane parallel to the load-bearing surface of a _____ unit is less than 75% of its gross cross-sectional area measured in the same plane.

The statement pertains to hollow units, which are construction materials characterized by their internal voids or cavities. These voids effectively reduce the net cross-sectional area of the unit because they occupy space that would otherwise contribute to the unit's overall structural integrity and load-bearing capacity.

In this context, the net cross-sectional area refers specifically to the area that can effectively carry loads, which is hindered by the presence of these voids. When we measure it in a plane parallel to the load-bearing surface, the presence of the hollow sections results in the net cross-sectional area being less than 75% of the gross cross-sectional area. This is significant in structural engineering and building inspection because it informs the ability of the material to support loads, affecting the overall design and safety of a structure.

The incorrect options involve solid units, which do not have voids and thus have a net cross-sectional area that can be equal to or greater than 75% of the gross area. Whole and block are broader terms that may refer to either solid or hollow materials, but the specific condition about the cross-sectional area applies distinctly to hollow units. Understanding this difference is crucial for inspections and ensuring that building materials meet required standards for safety and performance.