On mixing two colorless gases, dark brown gas is formed. These gases are

Correct option is C

The correct answer is (c) NO and O2

Explanation:
• Nitric oxide (NO) and Oxygen (O2) are both strictly colorless gases under standard conditions.
• When rapidly mixed in air, they spontaneously undergo a chemical reaction to form nitrogen dioxide (NO2), which is a distinctly dark brown, toxic gas.
• The balanced chemical reaction is: $2NO + O_2 \rightarrow 2NO_2$.

Information Booster:
• NO2 is a major air pollutant contributing heavily to the formation of photochemical smog and acid rain.
• It possesses a sharp, biting odor and exists in an equilibrium mixture with its colorless dimer, dinitrogen tetroxide (N2O4).

Additional Knowledge:
NO2 and O2 (Option a)
• NO2 is already a brown gas, so this violates the premise of mixing "two colorless gases" initially.
N2O and NO (Option b)
• Both are colorless gases, but mixing Nitrous oxide (N2O) with NO at room temperature does not yield a dark brown gas directly without further oxidation.
NH3 and HCl (Option d)
• Mixing ammonia (NH3) and hydrogen chloride (HCl) produces dense, solid white fumes of ammonium chloride (NH4Cl), not a brown gas.
None of the above/More than one of the above (Option e)
• Incorrect, as NO and O2 perfectly satisfy the chemical criteria.

So the correct answer is (c)