What kind of substance in public health continuously deforms under shear force?

In public health, a substance that continuously deforms under shear force is classified as a fluid. Fluids are characterized by their ability to flow and take the shape of their containers, and they can be either liquids or gases. When a shear force is applied, fluids will experience deformation and move, which is a fundamental property that distinguishes them from solids, which maintain a fixed shape under similar forces.

The distinction to consider here is that while all liquids are fluids, not all fluids are liquids. The term "fluid" encompasses both liquids and gases, which can also flow and deform under shear stress. Thus, the most inclusive and accurate term to describe the behavior under shear force is "fluid," as it correctly identifies the category that includes any substance, including those in the gaseous state.

The other choices—liquid form, flowing water, and discharge—are more specific examples or contexts within the broader category of fluids. However, they do not capture the general principle of continuous deformation under shear as comprehensively as the term "fluid" does.