ANS:C - both (a) and (b).

The correct answer is: "both (a) and (b)." Diffusivity in concentrated solutions differs from that in dilute solutions due to changes in both viscosity with concentration and the degree of ideality of the solution. (a) Viscosity with concentration: As the concentration of solute increases in a solution, the viscosity of the solution typically increases. This increased viscosity can hinder the movement of molecules and affect the rate of diffusion. Therefore, changes in viscosity with concentration can impact diffusivity in concentrated solutions. (b) Degree of ideality of the solution: In concentrated solutions, deviations from ideal behavior become more significant due to interactions between solute particles. These interactions can affect the movement of molecules and alter the diffusive behavior of the solution. Therefore, changes in the degree of ideality of the solution can also influence diffusivity in concentrated solutions. Therefore, both changes in viscosity with concentration and the degree of ideality of the solution contribute to the difference in diffusivity between concentrated and dilute solutions.