**Core Concept**
Nitric oxide (NO) is a crucial mediator of vasodilation, playing a significant role in the regulation of vascular tone and blood pressure. It is produced by the enzyme nitric oxide synthase (NOS) in the endothelium, the inner lining of blood vessels.

**Why the Correct Answer is Right**
Nitric oxide exerts its effects through the activation of guanylyl cyclase, leading to the production of cyclic guanosine monophosphate (cGMP). This increase in cGMP causes smooth muscle relaxation, resulting in vasodilation. The endothelium-derived relaxing factor (EDRF) was initially identified as a mediator of vasodilation and was later found to be nitric oxide.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because prostaglandins, while involved in vasodilation, are not the primary mediators of this effect. They are more commonly associated with inflammation and pain.
**Option B:** This option is incorrect because endothelin-1 is a potent vasoconstrictor, not a vasodilator. It is produced by the endothelium and causes smooth muscle contraction.

**Clinical Pearl / High-Yield Fact**
Nitric oxide is also involved in the regulation of platelet aggregation and adhesion, making it an important mediator in the prevention of thrombosis.

**Correct Answer:** C. Nitric oxide.