Question

what is the source of the carbon dioxide that is used in photosynthesis?
sunlight
the soil
the atmosphere
the plant

Explanation:

Plants take in carbon dioxide ($CO_2$) from the atmosphere through small openings called stomata on their leaves. Sunlight is the energy source for photosynthesis, not a source of $CO_2$. The soil provides water and minerals, not $CO_2$. The plant itself does not generate the $CO_2$ used in photosynthesis; it takes it from the external environment (atmosphere).

Answer:

the atmosphere