Final answer:
Carbonization is the most frequent fossilization process observed in the case of plant and invertebrate fossils. It involves the preservation of the organism as a thin film of carbon instead of the original organic matter.
Step-by-step explanation:
The most frequent fossilization process observed in the case of plant and invertebrate fossils is carbonization. Carbonization is a type of fossil preservation where the organism is preserved as a residual, thin film of carbon instead of the original organic matter. Leaves, fish, and graptolites are commonly preserved through carbonization. Compression of the original organism results in thin layers of carbon, and carbonization can also lead to the formation of coal.