Final answer:
Viscosity is lowest at the flattest part (option C) of the graph of shearing stress versus shearing strain, indicating less resistance to flow for the same amount of stress applied.
Step-by-step explanation:
The question pertains to the viscosity of fluids and its relationship with the rate of shearing strain. Viscosity can be thought of as a fluid's resistance to flow, and it is measured by observing how a fluid moves between two plates, one fixed and one in motion. From a graph of shearing stress versus shearing strain, viscosity is related to the slope of the graph. The flattest part of the graph, corresponding to option C, would indicate where the viscosity is lowest, as a shallow slope suggests less stress is needed for the same strain, indicating less resistance to flow (lower viscosity). In contrast, the steepest part of the graph (option A) would indicate higher viscosity. Viscosity being independent of the rate of shearing strain (option D) would mean that the fluid is a Newtonian fluid, where viscosity remains constant regardless of the applied stress.