ACTH signals the _______ to secrete cortisol.

The adrenal cortex is the correct answer because adrenocorticotropic hormone (ACTH) is produced by the pituitary gland and plays a vital role in stimulating the adrenal cortex to release cortisol. Cortisol is a key hormone involved in the body's response to stress and helps regulate various functions, including metabolism, immune response, and blood pressure.

This specific axis—known as the hypothalamic-pituitary-adrenal (HPA) axis—illustrates the interaction between these glands, where the hypothalamus releases corticotropin-releasing hormone (CRH) that prompts the pituitary gland to produce ACTH, which in turn stimulates the adrenal cortex to secrete cortisol.

In contrast, the liver, pancreas, and thyroid gland do not directly respond to ACTH for cortisol secretion. The liver plays a role in metabolic processes and glucose regulation, the pancreas is responsible for insulin and glucagon production that manage blood sugar levels, and the thyroid gland regulates metabolism through hormones like thyroxine and triiodothyronine. Each of these glands has distinct functions that do not involve the secretion of cortisol in response to ACTH.