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

The correct answer is Nitric oxide.

Explanation:

Nitric oxide (NO) is a colorless gas synthesized endogenously in the human body, primarily by vascular endothelial cells. It plays a critical role as a signaling molecule in the cardiovascular system.
The synthesis of nitric oxide occurs from the amino acid L-arginine by the action of an enzyme called nitric oxide synthase (NOS).

Once synthesized, nitric oxide diffuses into the adjacent vascular smooth muscle cells. Inside these cells, it activates the enzyme soluble guanylyl cyclase (sGC), which converts guanosine triphosphate (GTP) to cyclic guanosine monophosphate (cGMP).
The accumulation of cGMP leads to a cascade of events resulting in the relaxation of the smooth muscle cells surrounding the blood vessels. This relaxation process is known as vasodilation.

Vasodilation widens the lumen of the blood vessels, which decreases vascular resistance and consequently increases blood flow and lowers blood pressure throughout the body.
In contrast, nitrous oxide (commonly known as laughing gas) is primarily used as an anesthetic, while nitrogen dioxide and nitrogen pentoxide are toxic nitrogen oxides and are not synthesized by the body for physiological vascular regulation.