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

Nitric oxide

Nitric oxide (NO) is synthesized by endothelial cells and acts as a signalling molecule that relaxes vascular smooth muscle, causing vasodilation and increasing blood flow. Nitrous oxide (N₂O) is commonly known as laughing gas and is not the body's principal endogenous vasodilator. Nitrogen dioxide (NO₂) is a reactive air pollutant, while nitrogen pentoxide (N₂O₅) is not the physiological signalling molecule described. Therefore, option A is correct.