Which of the following is classified as a form of gas?

The correct answer is classified as a form of gas due to the fact that it encompasses a group of elements that exist in a gaseous state under standard temperature and pressure conditions. Oxygen, nitrogen, chlorine, and argon are all gases at room temperature, which is why they are collectively recognized in this context.

Water vapor is indeed a gas, but it is a specific state of water rather than a broad category of gas elements. Carbon monoxide, while also a gas, is not the broader representation of gases that includes multiple elements. Hydrogen peroxide is a liquid under normal conditions and does not fall under the classification of gases.

Therefore, the chosen answer is valid as it represents a category of substances known to exist in the gaseous state, making it a fitting description of a form of gas in the context of the question.