Final answer:
The source of carbon dioxide used in photosynthesis is the atmosphere. Green plants take in carbon dioxide from the air and combine it with water and sunlight to produce glucose and oxygen.
Step-by-step explanation:
The source of the carbon dioxide used in photosynthesis is the atmosphere (c). During photosynthesis, green plants take in carbon dioxide from the air and combine it with water and sunlight to produce glucose (a carbohydrate) and oxygen. This process is essential for plants to create their own 'food' and release oxygen into the atmosphere.