By which process do plants obtain carbon from the atmosphere? photosynthesis cellular respiration decomposition burning fossil fuels

The correct answer is - photosynthesis.

The process of photosynthesis is crucial for the development and existence of the plants. This process is so important for the plants because through it they manage to get energy from the sunlight and turn into a chemical energy. In order for that be possible though, the plants need carbon dioxide, and they take it in from the air, but also through their roots as well. While using the carbon dioxide in the process of photosynthesis, the plants release byproducts, such as the oxygen that they don't need in this process, which in turn is of great benefit for the animals and the humans,

a. i. only is correct.