Without green house effect, the average temperature of earth surface would be

1. A

   0°C

   (a) 0°C. Zero is the ice point of water, not the no-greenhouse planetary mean.

2. B

   –18°C

   (b) –18°C. Without the natural greenhouse effect Earth’s average surface would be about –18°C (255 K). The actual mean is about +15°C. That textbook figure is the official key.

3. C

   5°C

   (c) 5°C. Too warm for a no-greenhouse Earth and not the standard value.

4. D

   –20°C

   (d) –20°C. Close in magnitude but not the conventional –18°C figure used in Indian exam keys.

Summary. Official key is (b). Without greenhouse gases the global mean surface temperature would be about –18°C, not 0°C, 5°C or –20°C.