Non-acidic gases neither donate protons nor form acids in water.

Steps:

• Define non-acidic gases as those that do not produce H+ ions or acidic solutions when dissolved in water.
• Classify ammonia (NH3): reacts with water to form basic NH4OH, so non-acidic.
• Classify methane (CH4): inert hydrocarbon, no reaction with water, neutral and non-acidic.
• Check other gases: CO2 forms H2CO3 (acidic); NO2 forms HNO3/HNO2 (acidic).

Why A is correct:

• Both ammonia (basic, NH3 + H2O → NH4+ + OH-) and methane (neutral, no ionization) fail to produce acidic solutions.

Why the others are wrong:

• B: Carbon dioxide is acidic (CO2 + H2O → H2CO3).
• C: Nitrogen dioxide is acidic (2NO2 + H2O → HNO2 + HNO3).
• D: Both carbon dioxide and nitrogen dioxide form acids in water.

Final answer: A