Which gas do humans exhale that plants use for photosynthesis?

Humans exhale carbon dioxide, which is a critical component required by plants for photosynthesis. During this process, plants take in carbon dioxide from the atmosphere through small openings in their leaves called stomata. Using sunlight and chlorophyll, they convert carbon dioxide and water into glucose and oxygen. The production of glucose provides energy for the plant, while the oxygen released as a byproduct benefits humans and other living organisms by contributing to the air we breathe. Thus, carbon dioxide is essential for the photosynthesis process in plants, making it the correct answer to the question.