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

Stage efficiency in a leaching process depends on both (a) and (b):

• Time of contact between the solid and the solution: Longer contact times allow for more thorough extraction of solute from the solid phase into the liquid phase. Therefore, increasing the time of contact between the solid and the solution can improve the efficiency of the leaching process.
• Rate of diffusion of the solute through the solid and into the liquid: The rate at which the solute can diffuse from the solid phase into the liquid phase significantly influences the efficiency of the leaching process. Faster diffusion rates result in quicker extraction of the solute, leading to higher efficiency.

Together, these factors contribute to the overall efficiency of the leaching process. The vapor pressure of the solution is not directly related to the efficiency of the leaching process and therefore does not significantly affect stage efficiency in this context.