Which tip adjustment allows for more or less material flow in spray application?

Fluid adjustment is the correct choice because it directly controls the volume of material that is delivered through the spray gun. By turning the fluid adjustment knob, the painter can regulate how much paint or other material is being expelled from the gun, thereby influencing the thickness of the application. This is crucial for achieving the desired finish and ensuring that the material is applied evenly and adequately for the job at hand.

Other options have their specific purposes, but they do not directly change the amount of material that flows out of the gun. For instance, fan width adjustment affects the shape and spread of the spray pattern, allowing for either a wide or narrow application but not the quantity of material. Air pressure adjustment controls the speed at which the material is atomized and delivered but does not directly modify the fluid flow rate. Nozzle size adjustment influences the aerodynamics of the spray and can change flow characteristics but doesn't allow for fine control over the amount of product being used. Therefore, fluid adjustment is specifically designed to allow for more or less material flow, making it the correct answer.