Answer: Oxygen
Explanation:
Photosynthesis is the process by which plants make carbohydrates by the help of the sunlight, chlorophyll, water and other minerals.
Oxygen is the by product of this process. Oxygen is only produced by plants which is the basis for the respiration for all the organism on earth.
So, the correct answer is oxygen, which is only component produced by plants present in atmosphere. There is no other organism which can produce oxygen in atmosphere.