What defines a fluid's ability to flow through narrow spaces?

Viscosity is the property that defines a fluid's resistance to flow. It represents how thick or thin a fluid is; for instance, honey has a higher viscosity than water, meaning it flows more slowly. When fluid flows through narrow spaces, such as in pipes or small openings, the viscosity plays a critical role in determining how easily it can move. A fluid with low viscosity can flow freely and quickly, while a fluid with high viscosity flows more slowly and may struggle to move through tight spaces effectively. Thus, in this context, viscosity directly relates to a fluid's ability to navigate through narrow passages, making it the correct choice.

The other factors listed—density, fluid friction, and surface tension—do influence fluid behavior but not in the same way that viscosity does concerning flow through narrow openings. Density, for instance, is a measure of mass per unit volume and affects buoyancy rather than flow rate, while fluid friction relates to the resistance against fluid motion but is not a defining property of how easily a fluid can flow through confined spaces. Surface tension pertains more to the cohesive forces at the surface of a liquid and can influence how it interacts with other interfaces rather than directly describing flow characteristics in narrow spaces.