Answer: D. Rate of diffusion increases with two factors: (i) lower particle mass and (ii) higher temperature (greater average kinetic energy).
Compare relative molecular masses: \(M_r(\text{CO}_2) = 12 + 2 \times 16 = 44\); \(M_r(\text{C}_3\text{H}_6) = 3 \times 12 + 6 \times 1 = 42\). Propene is the lighter gas, so it diffuses faster than \(\text{CO}_2\) at any given temperature.
Higher temperature also speeds up diffusion. So the lightest gas at the highest temperature diffuses most rapidly: propene at 75 °C — option D. Options A and B (both \(\text{CO}_2\)) are heavier; option C (propene at 25 °C) is right gas but lower temperature, so slower than D.
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