Most osmotically active intracellular cation: (A) K+, (B) Na+, (C) Mg+2, (D) Protein
Answer is A. Potassium is the major intracellular cation, it helps to establish resting membrane potential in neurons and nerve fibers after membrane depolarization and action potential Ref:Guyton and Hall textbook of medical physiology 12th edition,page number:50,51,52