Question

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

Explanation:

Plants take in carbon dioxide ($CO_2$) from the atmosphere during photosynthesis. Fossil fuels are hydrocarbons and not directly in a form that plants can take up for photosynthesis. Dead organisms decompose and release carbon but not in a way that is directly taken up by plants during photosynthesis. Seawater can dissolve $CO_2$ but the primary reservoir that plants access during photosynthesis is the atmosphere.

Answer:

Atmosphere