Which gas is naturally occurring and forms a significant part of Earth's atmosphere?

The answer encompasses all the listed gases because they are all naturally occurring components of Earth's atmosphere. Each of these gases plays a unique role in various processes that sustain life and influence environmental conditions.

Argon, for instance, is the third most abundant gas in the atmosphere, making up about 0.93% of the air we breathe. It is a noble gas, which means it is chemically inert, and it does not readily react with other substances.

Oxygen is a critical gas for life, constituting approximately 21% of the atmosphere. It is essential for cellular respiration in most living organisms and plays a crucial role in combustion processes.

Methane, though present in smaller quantities (around 0.0002% of the atmosphere), is a significant greenhouse gas. It is naturally emitted from wetlands, as well as from geological sources and human activities. Methane's role in trapping heat makes it an important factor in discussions about climate change despite its relatively low concentration.

By recognizing that each of these gases is a natural component of our atmosphere, it becomes clear why the selected answer includes all of them as significant atmospheric constituents.