**Core Concept**
Aldosterone synthesis is a critical process in the renin-angiotensin-aldosterone system (RAAS), which regulates electrolyte balance and blood pressure. The key principle being tested here is the regulation of aldosterone secretion by the adrenal cortex.

**Why the Correct Answer is Right**
Aldosterone synthesis is stimulated by angiotensin II, a potent vasoconstrictor that acts as a hormone. Angiotensin II binds to the angiotensin II type 1 receptor (AT1R) in the zona glomerulosa of the adrenal cortex, leading to increased production of aldosterone. This hormone promotes sodium retention in the kidneys, which in turn increases blood volume and blood pressure.

**Why Each Wrong Option is Incorrect**
**Option A:** ACTH (Adrenocorticotropic hormone) primarily stimulates the production of cortisol and other glucocorticoids, not aldosterone.

**Option B:** Vasopressin, also known as antidiuretic hormone (ADH), regulates water reabsorption in the kidneys but does not directly stimulate aldosterone synthesis.

**Option C:** Progesterone is a sex hormone produced by the adrenal cortex, but it does not stimulate aldosterone production.

**Clinical Pearl / High-Yield Fact**
Remember the RAAS pathway: Renin → Angiotensinogen → Angiotensin I → Angiotensin II → Aldosterone → Increased blood pressure and sodium retention.

**Correct Answer:** C. Progesterone.