Which gas do plants primarily use for photosynthesis?

Plants primarily use carbon dioxide for photosynthesis because it is a key ingredient in the process they use to create their own food. During photosynthesis, plants take in carbon dioxide from the atmosphere through tiny openings in their leaves called stomata. They then combine it with water, which they absorb from the soil, using sunlight as energy to convert these raw materials into glucose (a type of sugar) and oxygen.

The oxygen produced is released back into the atmosphere as a byproduct, which is essential for the survival of most living organisms on Earth. Therefore, carbon dioxide is critical for the growth and energy needs of plants, making it the correct answer in this context.