**Core Concept**
The question is related to the baroreceptor reflex, which is a critical mechanism for maintaining blood pressure homeostasis. This reflex involves **baroreceptors** located in the walls of the **aortic arch** and **carotid sinus**.

**Why the Correct Answer is Right**
A detectable fall in blood pressure occurs when there is a significant decrease in **venous return**, leading to a decrease in **cardiac output**. This decrease is detected by **baroreceptors**, which then activate the **sympathetic nervous system** to increase heart rate and vascular tone, thereby maintaining blood pressure.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is not directly related to the baroreceptor reflex.
**Option B:** Incorrect as it is not a primary mechanism for detecting falls in blood pressure.
**Option C:** Incorrect because it is related to a different physiological process.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the baroreceptor reflex is a rapid-acting mechanism that helps to maintain blood pressure homeostasis, and its dysfunction can lead to **orthostatic hypotension**.

**Correct Answer:** D. 15-20% fall in blood pressure