Carbon leaves the atmosphere and enters the terrestrial biosphere through photosynthesis and dissolution. Thus, carbon enters bodies of water and plants, where it is incorporated into organisms as organic molecules. Then the different trophic levels will consume these organic molecules. Upon their death, the carbon will be recycled within the terrestrial biosphere.

Carbon leaves the atmospheric biosphere and enters the terrestrial biosphere through photosynthesis and dissolution into bodies of water. Recall that carbon dioxide is a reactant of photosynthesis, and it gets converted into glucose. 

When carbon enters into bodies of water through precipitation, it reacts with water to form carbonic acid. Carbonic acid then is absorbed into rocks in the sedimentary biosphere. Carbonic acid is very soluble in water, and is the form that most of carbon is in when dissolved in water.

The burning of fossil fuels has increased the amount of carbon in the atmosphere, contributing to the greenhouse effect and global warming. Burning fossil fuels mainly involves combustion reactions in which the fuel is oxidized, producing carbon dioxide and water. While the other answer choices may add to the carbon dioxide levels of the atmosphere, the question asks for a direct cause.

In the terrestrial biosphere, carbon can be found in all living and dead organisms, stored in the soil, and in inorganic forms. Inorganic carbon refers to carbon in minerals and ores. It is often said that life on earth is carbon-based due to its versatility and ubiquity.

Carbon that is contained in the terrestrial biosphere leaves and returns to the atmospheric biosphere through animal, plant, and soil respiration. These terms refer to the process by which food molecules (ie. glucose) are burned and converted to more usable forms of energy. The byproducts are often carbon dioxide, water, and heat. In animal respiration, carbon dioxide is produced during the transition step between glycolysis and Krebs cycle, and twice in the Krebs cycle.

Plants and animals receive carbon through photosynthesis and consumption, respectively.

Carbon from terrestrial organisms can enter into the oceanic biosphere and bodies of water through dead tissue, dissolution, or in the form of shells as calcium carbonate.

Industrialization has increased the burning of fossil fuels, such as coal and oil. This releases carbon from the geosphere into the atmosphere, increasing the carbon in the atmosphere. Carbon dioxide and methane are big contributors to the greenhouse effect and global warming.

Modern human activities greatly impact the carbon cycle. Increase in land usage decreases the presence of natural ecosystems and the ability of organisms to remove carbon from the atmosphere. Pollution damages organisms, which reenters the terrestrial biosphere and increases erosion. Deforestation directly removes carbon rom ecosystems and decreases the amount of carbon absorbed from the atmospheric biosphere.