Most abundant gas in atmosphere.

**The reasoning** Earth's atmosphere is about **78% nitrogen (N₂)** by volume. That makes it the clear winner. Oxygen comes second at about 21%, argon third at roughly 1%, and CO₂ is actually a trace gas at only about 0.04%. So the ranking is: Nitrogen > Oxygen > Argon > CO₂. Think of the atmosphere as a room with 100 people: 78 would be nitrogen molecules, 21 oxygen, 1 argon, and CO₂ wouldn't even fill one person's space! **Why the wrong options tempt you** **Oxygen (A)** — We breathe it constantly, so it *feels* like it should dominate. But we only need that 21% to survive; nitrogen is just along for the ride. **CO₂ (C)** — Gets tons of attention in climate discussions, making it seem abundant. Reality? It's trace amounts. **Argon (D)** — Most students haven't even heard of it, so it's easy to dismiss, but it actually beats CO₂! **Quick takeaway** When you breathe, remember: you're swimming through an ocean of nitrogen (78%), with just enough oxygen (21%) to keep you alive.