**Core Concept**
The question is testing the body's response to increased blood pressure, specifically the mechanism by which the blood pressure is regulated. The correct answer involves the baroreceptor reflex, a critical physiological mechanism that helps maintain blood pressure homeostasis.

**Why the Correct Answer is Right**
In response to raised blood pressure, baroreceptors in the walls of the carotid sinus and aortic arch are activated. These mechanoreceptors detect the increased stretch and send signals to the brainstem, which then activates the parasympathetic nervous system. The release of **acetylcholine** from the vagus nerve terminals acts on muscarinic receptors in the heart, causing a decrease in heart rate (negative chronotropic effect) and a reduction in cardiac output. This, in turn, helps to lower blood pressure. Acetylcholine also stimulates the release of nitric oxide from the endothelium, leading to vasodilation and further lowering of blood pressure.

**Why Each Wrong Option is Incorrect**
**Option B:** Adrenaline is not released in response to raised blood pressure; instead, it is released in response to stress or hypotension to increase blood pressure.
**Option C:** Dopamine is involved in the regulation of blood pressure, but it is not the primary neurotransmitter released in response to raised blood pressure. It is more involved in the modulation of renal and mesenteric blood flow.
**Option D:** Nor-adrenaline is released in response to hypotension, not raised blood pressure. It acts on alpha-adrenergic receptors to cause vasoconstriction, increasing blood pressure.

**Clinical Pearl / High-Yield Fact**
The baroreceptor reflex is a critical mechanism for maintaining blood pressure homeostasis. It is often tested in exams, and students should remember that the reflex involves the release of acetylcholine from the vagus nerve terminals in response to raised blood pressure.

**✓ Correct Answer: A. Acetylcholine**