Answer:
(a) Nitric oxide
Explanation:
Nitric oxide (NO) is a molecule that is synthesized in the human body and plays a crucial role in vasodilation, which is the widening of blood vessels. This process helps to increase blood flow and lower blood pressure. Nitric oxide is produced by endothelial cells lining the blood vessels and acts as a signaling molecule that relaxes the smooth muscles in the vessel walls.
Nitrous oxide (N2O), also known as laughing gas, is used as an anesthetic and analgesic.
Nitrogen dioxide (NO2) is a pollutant and a toxic gas.
Nitrogen pentoxide (N2O5) is a chemical compound that is not synthesized in the human body and is used in chemical synthesis.
Therefore, nitric oxide is the correct answer.
