It is known that the climate on earth has fluctuate in the past, but the present concern is that there will be a significant climate change because of a. population of people on earthb. addition of carbon dioxide to the atmospherec. addition of oxygen to the atmosphered. deterioration of the atmosphere

The correct answer is option b, that is, the addition of carbon dioxide to the atmosphere.  

If the concentration of carbon dioxide upsurges in the atmosphere due to the enhanced rate of outgassing, there will be a constant elevation in the global temperature. The elevating temperature and more dissolved carbon dioxide will result in enhanced weathering of crystal rocks as a consequence of faster rate of reactions and greater acidity.