ANS:A - D^0.5

In the context of mass transfer, the notation 𝐷0.5D0.5 refers to the square root of the mass diffusivity (𝐷D). Mass diffusivity (𝐷D) is a measure of how quickly a substance can diffuse through a medium, such as a gas or liquid. When the mass transfer coefficient (𝐾K) varies with mass diffusivity as 𝐷0.5D0.5, it means that the mass transfer coefficient is proportional to the square root of the mass diffusivity. Mathematically, this relationship can be expressed as: 𝐾∝𝐷K∝D​ This means that as the mass diffusivity increases, the mass transfer coefficient increases, but not in a linear manner. Instead, the increase follows the square root of the mass diffusivity. The square root relationship between mass transfer coefficient and mass diffusivity is often observed in systems where diffusion is the predominant mode of mass transfer, such as in gas-phase or liquid-phase diffusion processes.