Which of the following is NOT an example of a toxic gas?

Oxygen is a vital gas that is essential for life, and it plays a crucial role in various biological processes, including respiration in most living organisms. While it can be present in toxic amounts in certain extreme circumstances, it is not classified as a toxic gas under normal conditions. Instead, it is beneficial and necessary for sustaining life, as it supports cellular metabolism. In contrast, carbon monoxide and nitrogen dioxide are recognized as toxic gases that can pose serious health risks when inhaled. Hydrogen, although not inherently toxic, can be flammable and poses risks in certain environments. Thus, the identification of oxygen as the option that does not represent a toxic gas is based on its fundamental role in supporting life rather than causing harm under normal exposure conditions.