How is internal friction within a fluid quantified?

Viscosity is the correct choice because it is a key measure of a fluid's internal friction or resistance to flow. Essentially, viscosity quantifies how easily a fluid can deform under shear stress; the higher the viscosity, the greater the internal friction, making it more resistant to flow.

In practical terms, when a fluid with high viscosity (like honey) flows, it does so more slowly compared to a fluid with low viscosity (like water), which flows easily. Viscosity is crucial in understanding fluid dynamics, as it affects how fluids behave in various situations, including blood flow in the human body, which is particularly relevant in vascular ultrasound.

Density measures the mass per unit volume of a fluid and does not directly relate to its internal friction. Pressure refers to the force exerted by the fluid per unit area, while flow rate measures how much fluid passes through a given area in a specified time. Neither of these addresses the fluid's internal resistance to flow, making viscosity the appropriate and relevant measure for quantifying internal friction.