Final answer:
The most specific level of organization in both classification systems, which falls immediately under the genus, is the species.
Step-by-step explanation:
In both classification systems, the species is the most specific level of organization and is immediately under the genus. Therefore, the answer is C. Species. The taxonomic classification system, also known as the Linnaean system, categorizes organisms into a hierarchical structure that ranges from the broadest level at the domain to the most specific level at the species. Each level of classification groups organisms based on shared characteristics, with species being the most exclusive group consisting of individual organisms that can interbreed.