The correct answer is B. Iron oxides form when cyanobacteria produce oxygen via photosynthesis.
Iron oxides can be found in sedimentary rocks and are considered evidence for prehistoric photosynthesis because they typically form when oxygen is present in the environment, which is a byproduct of photosynthesis carried out by organisms, particularly cyanobacteria. When cyanobacteria evolved to perform photosynthesis, they released oxygen into the atmosphere, leading to the precipitation of iron oxides in ancient ocean environments. This process is a key indicator of the Great Oxidation Event, which significantly changed the composition of the Earth's atmosphere.