The correct choice is (c) Standard free energy change
Best explanation: Standard free energy change is referred to as the free energy change occurring at standard conditions, i.e., at 25°C, pH 7, 1 mol/L solute concentration and normal gas. Gibbs free energy is the energy that exist for performing work. Activation energy is the energy required to attain transition state. pKa is the pH at which half of the ions are ionized.