Final answer:
Kidneys adjust plasma volume and osmolarity by regulating water reabsorption, with the influence of hormones ADH and aldosterone.
Step-by-step explanation:
Kidneys compensate for changes in plasma volume and osmolarity by adjusting the rate of water reabsorption. The hormones ADH (antidiuretic hormone) and aldosterone play significant roles in this process. ADH increases water reabsorption in the collecting ducts, which can recover almost all of the water passing through them when the body is dehydrated. Aldosterone increases sodium reabsorption, which also promotes water reabsorption. However, the kidneys do not adjust water balance via secretion, since water is not secreted but rather filtered and then reabsorbed. Therefore, the correct answer to the question is 1) reabsorption only.