Final answer:
Oil shale is the organic sedimentary rock that consists of mud-sized particles and partially decayed organic matter from marine microorganisms.
Step-by-step explanation:
The organic sedimentary rock composed of mud-sized particles and partially decayed organic matter, usually derived from plankton and other marine microorganisms, is called oil shale. Sedimentary rocks form through various mechanisms, including the weathering and erosion of pre-existing rocks, resulting in clastic sediments like sandstone and mudstone, or through the precipitation of chemicals, forming non-clastic rocks such as limestone and salt. Organic sedimentary rocks are a specific type that form from the accumulation and lithification of organic debris, which can include plant or animal material. In the case of oil shale, this includes the significant presence of kerogen, a waxy organic compound that results from the compression and heating of the remains of microscopic marine organisms.