What property of a fluid is determined by its viscosity?

Viscosity is a measure of a fluid's resistance to flow and is a critical property that influences how fluids behave in various applications. It determines how easily a fluid can move and how it interacts with surfaces, particularly in confined spaces or narrow passages.

When considering small clearances, a fluid with lower viscosity will flow more readily and can penetrate those spaces with greater ease. Conversely, a fluid with higher viscosity will be more resistant to flow, making it less capable of moving into tight areas effectively. Therefore, viscosity is directly related to the fluid's ability to fill these spaces.

The other properties listed in the choices focus on aspects of fluids that are not directly governed by viscosity. Density under pressure pertains to how a fluid's mass per unit volume changes with increased pressure, while chemical stability refers to the fluid's resistance to chemical reactions over time. Temperature stability relates to a fluid's ability to maintain its properties under varying thermal conditions. None of these are influenced by viscosity in the way that the ability to penetrate small clearances is.