Which one of the following is synthesised in human body that dilates blood vessels and increases blood flow?

Nitric oxide (NO) is synthesized in the human body, primarily by endothelial cells lining blood vessels, from the amino acid L-arginine via the enzyme nitric oxide synthase. It acts as a powerful vasodilator by relaxing the smooth muscle in blood vessel walls, thereby increasing blood flow. This discovery led to the 1998 Nobel Prize in Physiology or Medicine and underlies the action of drugs like nitroglycerin and sildenafil (Viagra). Nitrous oxide is an anesthetic, while nitrogen dioxide and nitrogen pentoxide are toxic pollutants not synthesized in the body.