Answer:
Proterozoic
Step-by-step explanation:
Stromatolites appear on Earth in the end of Proterozoic era because Proterozoic extended from 2500 million years to 542 million years. The stromatolites were present on earth about 542 million years ago. The stromatolites present in large number on the sea shores around the world and are considered as the Earth's first visible life form which can be seen through naked eye. These stromatolites were the first known organisms in which photosynthesis occurs and produce free oxygen..