Question

chemical reaction predictions quick check
why are iron oxides in rocks evidence for prehistoric photosynthesis? (1 point)
iron oxides form when cyanobacteria use carbon dioxide for photosynthesis.
iron oxides form when oxygen, a product of photosynthesis, is in the atmosphere.
iron oxides form when carbon dioxide, a reactant of photosynthesis, is in the atmosphere.
iron oxides form when cyanobacteria produce oxygen via photosynthesis.

Explanation:

Iron oxides (rust) form through oxidation reactions. Photosynthesis by cyanobacteria (and later plants) released oxygen ($\ce{O2}$) as a by - product. Oxygen in the atmosphere can react with iron - containing minerals in rocks to form iron oxides. Carbon dioxide ($\ce{CO2}$) is a reactant in photosynthesis but does not directly form iron oxides. Also, the key is the presence of oxygen (a product of photosynthesis) in the atmosphere for iron oxidation, not just the production of oxygen by cyanobacteria (as the oxygen needs to be in the atmosphere to react with iron in rocks).

Answer:

Iron oxides form when oxygen, a product of photosynthesis, is in the atmosphere.