The correct answer is - d. atmosphere.
The living organism on the Earth have had a key role in the structure of the lower atmosphere, or rather the troposphere, as we now know it. The biggest influence had been by the plants. The plants started to use carbon dioxide from the atmosphere for the process of photosynthesis, but since they only used the carbon, they were releasing the oxygen back. As the number of plants grew, more and more oxygen in the atmosphere had been released by them, thus making the atmosphere suitable for the animals to come on the surface and start live on it.
Nowadays, the human species has also been heavily involved with its activity in the change of the atmosphere, and that is mainly with the much increased emissions of carbon dioxide and methane into it.
