Which hormone is primarily responsible for water retention in the kidneys?

Antidiuretic hormone (ADH), also known as vasopressin, plays a critical role in regulating water balance in the body by promoting water retention in the kidneys. When ADH is released from the posterior pituitary gland, it acts on the kidney's collecting ducts, increasing their permeability to water. This allows more water to be reabsorbed back into the bloodstream, thereby reducing urine output and helping to concentrate urine. This mechanism is particularly important in maintaining proper hydration status and blood osmolarity.

In contrast, adrenaline, insulin, and cortisol have different primary functions that do not directly target water retention. Adrenaline primarily prepares the body for a 'fight or flight' response, while insulin manages glucose levels in the blood, and cortisol is involved in metabolism and the body's response to stress. Thus, among the hormones listed, ADH is uniquely positioned to directly influence water retention in the kidneys, making it the correct answer.