**Core Concept**
The composition of normal dry air in the atmosphere is primarily made up of nitrogen, oxygen, and trace amounts of other gases. Understanding the relative concentrations of these gases is crucial for various medical and environmental applications.

**Why the Correct Answer is Right**
Nitrogen makes up approximately 78% of the Earth's atmosphere, with oxygen being the second most abundant gas at around 21%. The remaining 1% consists of trace gases, including carbon dioxide, neon, helium, methane, and hydrogen. The high concentration of nitrogen is due to the fact that it is a byproduct of the nitrogen cycle, which involves the conversion of nitrogen gas to ammonia and other nitrogen compounds by microorganisms.

**Why Each Wrong Option is Incorrect**
**Option A:** Carbon dioxide is present in the atmosphere but at a much lower concentration (around 0.04%) compared to nitrogen.
**Option B:** Oxygen is the second most abundant gas in the atmosphere but not the most abundant.
**Option D:** Trace gases like neon, helium, and hydrogen make up a very small fraction of the atmosphere, less than 1%.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the Earth's atmosphere is dynamic and can be affected by human activities, such as burning fossil fuels, which release large amounts of carbon dioxide into the atmosphere, contributing to climate change.

**Correct Answer: D. Nitrogen. Nitrogen is the most abundant gas in the Earth's atmosphere, making up approximately 78% of the dry air.**