The fossil record suggests that stromatolites were more comm

The fossil record suggests that stromatolites were more common early in Earth\'s history than they are today. How do stromatolites form? Layered rock structures form as calcium carbonate and sediment in the water adhere to the sticky covering produced by generations of microbial mats. Sediment washed downriver in a flood blankets the microorganisms on the river bottom, killing the organisms and trapping their remains in a layer of rock. Calcium carbonate extracted from seawater by small marine animals forms rock structures with cavities in which the animals live. Shells and sand are compressed together in warm, shallow seas, forming hollow rock structures that may provide shelter for microorganisms.

Solution

The answer would be the first option i.e stromatoliths are formed from layered rock structure that form calcium carbonate and sediment in the water ahere to the sticky covering produced by generation of microbial mats.

Actually, stromatoliths  are layered bio-chemical structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by microbial mats of microorganisms. This could be cyanobacteria (a photosynthetic bacteria) or other nitrogen fixing bacteria. They form colonies which ultimately results in filaments, sheets, or even hollow balls. Fossilized stromatolites provide ancient records of life on Earth by these remains, some of which may date from 3.7 billion years ago.