Answer: Carbon is used for photosynthesis by plants and oxygen is used for the respiration by animals.
Step-by-step explanation:
Plants use carbon dioxide for photosynthesis and releases oxygen gas as a product.
Oxygen which is released is used by the animals for their cellular respiration and releases carbon dioxide which is again used by the plants for photosynthesis.
Hence, carbon is used for photosynthesis by plants and oxygen is used for the respiration by animals.