Answer:
c. Light
Explanation;
In 1779 Jan Ingenhousz found that how the plants exchange CO2 and Oxygen. He discovered the photosynthesis.
He showed that the green parts of plants release oxygen bubbles in the presence of light.
He also showed that the plants release the CO2 during the night and the amount of CO2 released during the night is less than the amount of oxygen released during the daylight. So plants also use some mass from soil.