Final answer:
Stromatolites are sedimentary structures formed by bacteria within microbial mats, representing some of the earliest forms of life on Earth.
Step-by-step explanation:
Microbial mats such as stromatolites are examples of c) Bacteria. Stromatolites are sedimentary structures that form when minerals are precipitated out of water by prokaryotes in a microbial mat. Microbial mats are multi-layered sheets composed primarily of bacteria and archaea. These mats, which can be found in places like the Anza-Borrego Desert State Park in California, are one of the earliest records of life on Earth and may hold significant information about the origins of life and the search for extraterrestrial life.