Plants obtain carbon primarily from carbon dioxide (CO₂) in the atmosphere through a process called photosynthesis. During photosynthesis, plants take in carbon dioxide through small openings in their leaves called stomata. Using sunlight, they convert CO₂ and water into glucose and oxygen. The glucose serves as an energy source for the plant, while the oxygen is released into the atmosphere.
So, the correct response to how plants obtain carbon is from carbon dioxide in the atmosphere.