Answer: The correct option is A.
Explanation: Plants produce their food by the process of photosynthesis.
Photosynthesis is a process in which plants use energy from sun, carbon dioxide from air and water to produce glucose and oxygen.
Equation for photosynthesis is given by:
[tex]Sunlight+6CO_2+6H_2O\rightarrow C_6H_{12}O_6+6O_2[/tex]
Plants obtain carbon in the form of carbon dioxide from the air.
Thus, the correct option for this question is A.