Final answer:
The rock described is limestone, which is composed of calcite and contains microscopic fossils of marine organisms.
Step-by-step explanation:
The rock that you have described, composed of calcite with microscopic fossils of marine organisms and reacting with acid, is called limestone. Limestone is a sedimentary rock that is primarily made up of calcite, which is a mineral form of calcium carbonate. It commonly contains fossils of marine organisms, such as shells and microscopic organisms.