Which type of stress is defined as the force per unit area acting perpendicular to the surface?

The correct choice is the type of stress that is defined as the force per unit area acting perpendicular to the surface, which is compressive stress. Compressive stress occurs when a material is subjected to forces that push inward, causing the material to reduce in size along the axis of the load. This type of stress is crucial in various applications, such as in structural engineering where columns and beams must withstand loads effectively.

Understanding compressive stress is important because it directly relates to the material's ability to withstand axial loads without failing. In materials science and engineering, knowing how different types of stress affect material behavior helps in designing safer and more efficient structures.

The other types of stress mentioned—shear stress, tensile stress, and bending stress—refer to different loading scenarios. Shear stress involves forces that cause layers of material to slide past each other, tensile stress relates to forces that pull material apart, and bending stress indicates the stresses that occur when a material is subjected to bending moments. Each plays a distinct role in the behavior of materials under different loading conditions, but they do not fit the definition of force per unit area acting perpendicular to a surface as compressive stress does.