Final answer:
In the carbon cycle, producers obtain carbon from the atmosphere through the process of photosynthesis, converting CO2 into glucose.
Step-by-step explanation:
In the carbon cycle, producers such as plants and photosynthetic bacteria get their carbon primarily from the atmosphere. These autotrophs capture carbon dioxide (CO2) from the air and convert it into glucose during the process of photosynthesis. This organic compound becomes the basis for building more complex organic molecules necessary for life. Marine autotrophs can also use dissolved carbon dioxide in the form of carbonic acid from the hydrosphere. Thus, the correct answer to the question is A) the atmosphere.