**Core Concept**
The zona glomerulosa is the outermost layer of the adrenal cortex, responsible for producing a specific type of steroid hormone. This hormone plays a crucial role in maintaining electrolyte balance and blood pressure.

**Why the Correct Answer is Right**
The zona glomerulosa produces aldosterone, a steroid hormone that acts on the kidneys to increase sodium reabsorption and potassium excretion. This results in increased blood volume and blood pressure. Aldosterone acts through the mineralocorticoid receptor, which is found in the collecting ducts of the kidneys. The increased sodium reabsorption also leads to increased water reabsorption, further increasing blood volume and blood pressure.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the zona glomerulosa does not produce cortisol, which is produced by the zona fasciculata. Cortisol is involved in stress response and glucose metabolism.
**Option B:** This option is incorrect because the zona reticularis produces androgens, such as dehydroepiandrosterone (DHEA), which are involved in the development of male characteristics. These hormones are not produced by the zona glomerulosa.
**Option C:** This option is incorrect because the zona glomerulosa does not produce insulin, which is produced by the pancreas. Insulin is involved in glucose metabolism and is not related to the zona glomerulosa.

**Clinical Pearl / High-Yield Fact**
Aldosterone is often referred to as the "electrolyte hormone" because of its role in maintaining electrolyte balance in the body. This is particularly important in patients with heart failure or liver disease, where aldosterone levels may be elevated, leading to fluid overload and hypertension.

**Correct Answer: D. Aldosterone.