Oxygen concentration in Earth’s atmosphere is roughly?

Correct — B. By volume, dry air is about 78.08% nitrogen, 20.95% oxygen, 0.93% argon and 0.04% carbon dioxide.

20.95% rounds to 21%.

A quick check rules out the higher options: nitrogen alone takes 78.08%, leaving 21.92% for all other gases together, so oxygen cannot reach 24% or 28%. That is option (b).

Why the others are wrong

• (a)
  18% — 18% is below the real figure. Dry air holds 20.95% oxygen by volume, which rounds to 21, not 18.
• (c)
  24% — 24% is impossible for dry air. Nitrogen's 78.08% leaves just 21.92% for oxygen, argon, carbon dioxide and every trace gas combined.
• (d)
  28% — 28% overshoots further still. With nitrogen at 78.08%, all the remaining gases together make up just under 22% of dry air.

Dry air is almost entirely three gases: nitrogen (78.08%), oxygen (20.95%) and argon (0.93%). Carbon dioxide, at about 0.04%, counts as a trace gas despite its role in the greenhouse effect.

Water vapour is left out of these figures because it varies. NCERT puts it at up to four per cent of the air by volume in the warm, wet tropics, and under one per cent over deserts and polar regions.

Key facts

• Dry air by volume: nitrogen 78.08%, oxygen 20.95%, argon 0.93%, carbon dioxide about 0.04%.
• Argon is the third most abundant gas in dry air, well ahead of carbon dioxide.
• Water vapour ranges from about 4% of air by volume in the humid tropics to under 1% over deserts and polar regions (NCERT Class XI).

Common traps

• Forgetting that nitrogen's 78% caps oxygen below 22%, which rules out 24% and 28% at once.

No directly related past PYQ was found.