Correct option (c) oceans
Sea water contains 50 times more CO2 than air. i.e. about 70% of total global carbon is found in oceans. This is in form of carbonates and bicarbonates. The atmosphere is the source of CO2 which is utilised by plants in photosynthesis reduced to form carbon compounds. The total mass of carbon currently in the atmosphere is about 7 x1017 g, i.e. 700000 million tonnes. Oceans regulate the CO2 content in the atmosphere and thus, play a very important role.