Diffusivity in a system is equivalent to which of the following?

Diffusivity in a system refers specifically to the ability of a substance to spread out or diffuse through another medium. The correct answer, which identifies diffusivity, is the diffusion coefficient. The diffusion coefficient quantifies how fast a substance diffuses, essentially linking the rate of mass transfer to the concentration gradient.

When considering the relationship between the diffusion coefficient and the driving forces of diffusion, it becomes clear that the diffusion coefficient is fundamental to understanding the rate at which diffusion occurs under specific conditions. A higher diffusion coefficient indicates that a substance will diffuse more quickly in a given environment, while a lower coefficient suggests slower diffusion.

Other options such as mass flow rate, concentration gradient, and pressure coefficient do not represent diffusivity itself. Mass flow rate describes the amount of mass moving through a given area over time, concentration gradient refers to the difference in concentration of a substance across a distance, and pressure coefficient relates to how pressure affects flow but does not specifically pertain to the process of diffusion in the same direct manner as the diffusion coefficient does. Therefore, the diffusion coefficient is fundamentally linked to the concept of diffusivity in a system.