Which of the following gases is extremely toxic?

• A. Carbon dioxide
• B. Argon
• C. Helium
• D. Carbon monoxide

Answer: (D)

Carbon monoxide is an extremely toxic gas because it is colorless, odorless, and tasteless, making it particularly dangerous as it can go undetected until it causes severe harm or even death. When inhaled, carbon monoxide binds with hemoglobin in the blood to form carboxyhemoglobin, which significantly reduces the blood's ability to carry oxygen. This can lead to symptoms such as headaches, dizziness, confusion, and ultimately, unconsciousness or death if exposure continues.

In contrast, the other gases listed—carbon dioxide, argon, and helium—do not pose the same level of acute toxicity. Carbon dioxide is a naturally occurring component of the atmosphere and is not toxic in ordinary concentrations, though it can be dangerous in high levels due to asphyxiation. Argon is an inert gas that does not react chemically with the body, posing no toxic threat, and helium is a non-toxic, lighter-than-air gas that also does not have harmful effects on the body in normal uses. Hence, carbon monoxide stands out distinctly as the most hazardous among the choices provided.