The correct choice is (c) Isoelectric point
To explain I would say: Enzymes are amphoteric in nature. As the pH conditions vary, they may behave as acids or bases. But, there is particular pH of an enzyme at which the net charge on the molecule becomes zero. This point is called as isoelectric point. At this point, enzymes will have minimal solubility in aqueous solutions. pKa is defined as the pH at which half of the groups are ionized. Km is the concentration of the substrate at which half of the enzyme are in bound state. The time taken to reduce the activity to half of its original activity is called half-life.