In rheology, fluids are classified by how their viscosity (resistance to flow) behaves when you stir or push them harder (i.e., change the shear rate). Newtonian fluids are the simplest case.
Step 1: Define rheology terms. Rheology studies the flow of fluids. Two key types are Newtonian and non-Newtonian flow.
Step 2: Understand viscosity and shear rate. Shear rate is how fast fluid layers move relative to each other. Viscosity is the resistance to that flow.
Step 3: Newtonian fluids follow a simple rule. For Newtonian fluids (like water, glycerin), viscosity stays the same no matter how fast you shear them. The ratio of shear stress to shear rate is always constant.
Step 4: Dismiss the wrong options. Option 1 (viscosity increases with shear rate) describes shear-thickening (dilatant) fluids. Option 2 (viscosity decreases) describes shear-thinning (pseudoplastic) fluids. Option 4 (yield value) describes plastic fluids such as Bingham plastics.
Step 5: Conclude. Newtonian flow means the viscosity is independent of shear rate. That is the defining characteristic.
Answer: Option (3) — Constant viscosity irrespective of shear rate