In which of the following concentration cell, cell reaction will occur spontaneously?

To answer this question, we first need to understand what a concentration cell is. A concentration cell is a type of electrochemical cell where the two electrodes are the same material, the electrolytes on the two half-cells involve the same ions, but the electrolyte concentrations are different.

In a concentration cell, the cell reaction will occur spontaneously if the concentration of the electrolyte in one half-cell is greater than the concentration in the other half-cell. This is because the reaction will naturally want to move towards a state of equilibrium, where the concentrations are equal.

So, without specific examples of concentration cells, the general answer is that a cell reaction will occur spontaneously in a concentration cell where the electrolyte concentrations are unequal. The reaction will proceed from the area of higher concentration to the area of lower concentration until equilibrium is reached.