Question

if the number of photosynthetic organisms on earth decreased drastically, which would happen as a result?
less water would be available on earths surface.
less solar energy would reach earths atmosphere.
more oxygen gas would be found in earths atmosphere.
more carbon dioxide gas would be found in earths atmosphere.

Explanation:

Photosynthetic organisms (like plants) perform photosynthesis, which uses carbon dioxide ($\ce{CO_2}$) and produces oxygen ($\ce{O_2}$). If their number decreases drastically:

• Option 1: Photosynthesis doesn't directly control surface water availability, so this is incorrect.
• Option 2: Photosynthetic organisms don't affect solar energy reaching the atmosphere; solar energy is from the Sun, so this is incorrect.
• Option 3: With fewer photosynthetic organisms, less $\ce{O_2}$ is produced, so atmospheric $\ce{O_2}$ would decrease, not increase. This is incorrect.
• Option 4: Since photosynthesis consumes $\ce{CO_2}$, fewer photosynthetic organisms mean less $\ce{CO_2}$ is used. So, more $\ce{CO_2}$ would remain in the atmosphere. This is correct.

Answer:

D. More carbon dioxide gas would be found in Earth’s atmosphere.