Which gas is not a toxic gas?

Helium is not a toxic gas, making it the correct choice in this scenario. It is a noble gas that is chemically inert, which means it does not react with other substances and does not pose a threat to health in the concentrations typically encountered in the environment. In fact, helium is often used for various applications, including filling balloons and as a cooling medium in cryogenics due to its non-toxic nature.

On the other hand, gases like hydrogen sulfide, sulfur dioxide, and carbon dioxide can be harmful or toxic under certain conditions. Hydrogen sulfide is particularly dangerous, as it has a characteristic rotten egg odor and can be lethal at high concentrations. Sulfur dioxide is a respiratory irritant and can cause serious health issues with prolonged exposure. Carbon dioxide, while not toxic in small amounts, can become hazardous in high concentrations by displacing oxygen in the air, leading to asphyxiation.

This distinction highlights why helium stands out as the only non-toxic gas among the options provided.