What gas do plants take in during photosynthesis?

During photosynthesis, plants primarily take in carbon dioxide. This process occurs in the chloroplasts of plant cells, where carbon dioxide from the atmosphere combines with water and light energy to produce glucose and oxygen. The general equation for photosynthesis can be written as:

6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂

In this equation, the carbon dioxide (CO₂) serves as a crucial reactant that plants convert into glucose, which they use as energy. The oxygen is released as a byproduct, which is beneficial for other living organisms. Thus, carbon dioxide is integral to the photosynthetic process, making it the correct answer. This highlights the importance of plants in the ecosystem as they contribute to the carbon cycle and support life by providing oxygen.