Which of the following serves as source of carbon to plants?

Answer: Option A

Atmosphere CO₂ serves as source of carbon for plants.
Plants take up carbon dioxide from the atmosphere and convert it into food, forests and other similar ecosystems are considered to be some of the planet's most important carbon sinks.
Carbon dioxide during photosynthesis and much of this carbon dioxide is then stored in roots, permafrost, grasslands and forests. Plants and the soil then release carbon dioxide when they decay. Other organisms also release carbon dioxide as they live and die.