The anti- diuretic hormone is a hormone that controls the kidneys. This is the hormone that regulates the balance of fluid in the blood. When the concentration of fluids in the blood is falling below a particular limit, the blood becomes more concentrated and when the concentrated blood reaches the brain. The brain releases more ADH into the blood stream
When the ADH reaches the kidneys, it causes the kidneys to reabsorb the water and produce more concentrated urine. When the brain finds that the fluid levels are normal and the plasma is dilute, the releasing of the ADH decreases gradually.
Hence the option fluid is the right answer