Which gas is known to kill by replacing oxygen in the blood?

The correct answer is Carbon Monoxide. This gas is particularly dangerous because it binds to hemoglobin in red blood cells more effectively than oxygen does, which can impede the body's ability to transport oxygen throughout the system. When inhaled, carbon monoxide can displace oxygen in the blood, leading to hypoxia, which can cause symptoms ranging from headaches and dizziness to confusion, loss of consciousness, and ultimately death if exposure continues.

In contrast, nitrogen dioxide, while harmful as a respiratory irritant and contributor to air pollution, does not displace oxygen in the blood in the same lethal manner. Hydrogen sulfide is also toxic and can affect the body's processes, but its mechanism differs and primarily involves disrupting cellular respiration at the biochemical level. Methane is generally non-toxic and, while it can pose risks related to asphyxiation in high concentrations by displacing oxygen in enclosed spaces, it does not have the same direct impact on hemoglobin as carbon monoxide does.