The carbon cycle is one of the most important cycles to living organisms. Carbon is one the most abundant elements on earth, helping to form molecules such as sugars, lipids, and proteins. There is a constant exchange of carbon from the abiotic and biotic environmental elements to the atmosphere. The breakdown of glucose is known as cellular respiration, and creates the byproduct carbon dioxide. This exhaled carbon dioxide is the method by which humans return carbon to the carbon cycle. Plants capture this carbon dioxide and use it to make sugars in a process called photosynthesis. As organisms die, they decompose and get compressed by soil, sand, or ice. These remains result in stored fossil fuels, which will be utilized by plants and extracted by humans for energy. Their extraction can cause a buildup of carbon dioxide in the atmosphere, leading to global warming.

The carbon dioxide that is released and then absorbed by plants is part of the carbon cycle because carbon dioxide is made up of a carbon atom and two oxygen atoms. The carbon cycle involves transfer of carbon from organic sources (decaying animals and plants), to the soil as fossil fuels and plant nutrients, to the air via plant absorption and fossil fuel burning, and back to organic sources as plants consume carbon dioxide in photosynthesis and animals consume plants.

Animals can release carbon through respiration via exhalation of carbon dioxide into the atmosphere. Animals can also release carbon by decomposition, which breaks down organismal waste and dead organisms, and puts the carbon into the soil.

Plants use photosynthesis to capture carbon dioxide, and can store carbon via carbon fixation. Humans can burn wood and fossil fuels into the atmosphere, but most animals cannot release carbon via such processes.

There are several ways that humans impact the carbon cycle.  One of these is the burning of fossil fuels (often associated with driving cars), which releases carbon dioxide into the atmosphere. It is important to remember that other human activities also impact the carbon cycle. The cutting down of trees reduces the amount of  that can be taken out of the atmosphere.  One human impact that is not frequently referenced is the amount of methane gas () released into the atmosphere by cattle farms, which is much harder to take out of the air than .

A forest fire will destroy the animals and plants of an ecosystem, but it does not deplete that ecosystem of minerals. Carbon, specifically, will rise into the atmosphere as carbon dioxide . Since this is a gas, it will rise into the atmosphere. Any other minerals will return to the ground as ash. 

All the carbon in organisms was originally obtained by plants from the earth's atmosphere. Plants fix carbon in the form of carbon dioxide during photosynthesis. Then, any animal that comes along and eats the plant gets the carbon too. 

While not all organisms are capable of all three means of moving carbon to different stores, there are organisms that do respire, photosynthesize and upon death some will sink into the ocean's bottom and become sediment. All four are methods by which carbon is moved through the biosphere into other stores. Respiration and decomposition release carbon containing compounds into the atmosphere, and decomposition also releases carbon into the soil and ocean. Sedimentation allows carbon trapped in the bodies of phytoplankton and other micro marine photoautotrophs to be eventually moved by geological forces into the lithosphere of the Earth. Photosynthesis is in generally a method by which solar light energy is converted to chemical energy stored in the form of glucose a six carbon sugar using carbon dioxide and water as substrates. 

All of the other choices asides from the ocean floor are major carbon stores. The atmosphere contains large amounts of carbon dioxide despite composing just a fraction of a percent of the Earth's atmosphere. The lithosphere contains large amounts of coal, oil and natural gas all of which are various mixtures of carbon containing compounds. The ocean dissolves and stores large amount of the atmosphere's carbon dioxide and the biosphere is all inclusive of living organism which are carbon based and contain a wide variety of carbon compounds. 

Only three of the five answers listed move carbon from the lithosphere to the atmosphere and of these the burning of fossil fuels moves the majority of the carbon that is moved from the lithosphere to the atmosphere. 

Due to cell structure containing cell walls composed of cellulose made of the densely interwoven sheets of the six-carbon sugar glucose plants sequester large amounts of carbon in their cell walls.