Which of the following has the greatest effect on plasma osmolality?

Among the options, vasopressin (also called ADH or anti-diuretic hormone) has the greatest effect on plasma osmolality. Two primary system are especially involved in regulating the concentration of sodium and osmolarity of extracellular fluid: The osmoreceptor-ADH system and The thirst mechanism Although aldosterone decreases the rate of sodium excretion by the kidneys, the concentration of sodium in the extracellular fluid rises only by a few mEqs. The reason for this that when sodium is reabsorbed by the tubules, simultaneous absorption of equivalent amounts of water occurs. So, aldosterone does not cause a significant change in the plasma osmolality. Ref: Guyton and Hall 13th edition Pgno: 231, 232