Question

which reservoir holds carbon dioxide that can be taken up by plants during photosynthesis?
dead organisms
seawater only
atmosphere
fossil fuels

Explanation:

Plants take in carbon dioxide ($CO_2$) from the atmosphere during photosynthesis. Dead organisms decompose and release carbon over time but not directly for photosynthesis in the way the question asks. Seawater can hold carbon (e.g., as dissolved $CO_2$ or carbonates), but the primary source plants directly take up is the atmosphere. Fossil fuels are a long - term carbon reservoir and are not directly used by plants for photosynthesis (they need to be burned first to release $CO_2$ into the atmosphere).

Answer:

Atmosphere