**Core Concept**
The Earth's atmosphere is composed of various gases, each with distinct properties and concentrations. Understanding the relative abundance of these gases is crucial in the context of environmental science and public health.

**Why the Correct Answer is Right**
Argon is the third most abundant gas in the Earth's atmosphere, making up approximately 0.934% of the atmosphere by volume. This is due to the fact that argon is a noble gas, which means it is chemically inert and does not react with other elements to form compounds. As a result, argon has been present in the atmosphere since the Earth's formation and has not been significantly depleted or altered over time.

**Why Each Wrong Option is Incorrect**
**Option B:** Carbon dioxide is a greenhouse gas that plays a crucial role in the Earth's climate system. However, it is present in much smaller concentrations than argon, making up only about 0.04% of the atmosphere by volume.

**Option C:** Hydrogen is a highly reactive gas that is not typically found in its elemental form in the atmosphere. It is often bound to other elements, such as oxygen (water vapor) or carbon (methane).

**Option D:** Methane is a potent greenhouse gas that is present in the atmosphere at much lower concentrations than argon. It is primarily produced through biological processes, such as the digestion of ruminant animals or the decay of organic matter.

**Clinical Pearl / High-Yield Fact**
The relative abundance of gases in the atmosphere has significant implications for environmental health and climate change. Understanding the properties and concentrations of these gases can help inform public health policy and mitigate the impacts of climate change.

**✓ Correct Answer: A. Argon**