Hello, I am Professor White and it seems to me that you are having trouble understanding the carbon cycle based off of your recent exam scores. Don't worry, we'll catch you up to speed.
To begin linearly, carbon exists in the form of carbon dioxide gas (CO2) in the atmosphere before being removed from it through a process called photosynthesis and used to help make food for plants.
Moving from the Atmosphere to Plants
glucose: simple sugar that plants produce through photosynthesis
With the carbon being used to make the food for plants, it is then passed on through the food chain onto (i.e. herbivores eating the plants, carnivores eat the herbivores, etc.).
Moving from Plants to Animals
Moving from Animals to the Atmosphere
Through respiration, animals are able to take in oxygen and release CO2 into the atmosphere.
Edit: there were no skeletons of animals so I used what I had instead
Carbon can transfer from organisms into the ground either from animal droppings or if the organism dies. On rare occasions, the dead are buried under the ground and are subjected to high pressures; over millions of years, they can turn into fossil fuels.
Moving from Plants & Animals to Ground
When us humans burn fossil fuels to power electricity plants and factories, heat our homes, run our vehicles, etc., most of the excess carbon enters the atmosphere as carbon dioxide gas.
Moving from Ground to the Atmosphere
Just like the Earth's other bodies of water, the oceans soak up carbon from the atmosphere wherever air meets water (i.e. whenever wind causes waves and turbulence).