Which hormone is secreted by the posterior pituitary gland in response to increased plasma osmolarity?

The posterior pituitary gland secretes antidiuretic hormone (ADH), also known as vasopressin, in response to increased plasma osmolarity. When plasma osmolarity rises, it indicates a higher concentration of solutes in the blood, often signaling dehydration or a lack of water. In response, specialized osmoreceptors in the hypothalamus detect this change and stimulate the release of ADH from the posterior pituitary.

ADH primarily acts on the kidneys to promote water reabsorption, effectively concentrating the urine and reducing water excretion. This process helps to dilute the plasma and restore osmotic balance, thus regulating fluid balance and blood pressure.

The other hormones listed, while important in various physiological processes, do not specifically respond to increased plasma osmolarity in the same way. For instance, aldosterone regulates sodium and potassium levels, oxytocin is primarily involved in childbirth and lactation, and cortisol functions in stress response and metabolism but does not directly act to manage osmolarity. This makes antidiuretic hormone the accurate answer for this context.