Final answer:
The species is the taxonomic category that includes the fewest number of species, as it is the most specific and exclusive level in the Linnaean system of classification.
Step-by-step explanation:
The taxonomic category that includes the fewest number of species is the species itself. According to the Linnaean system of classification, organisms are classified into a hierarchy of groups, or taxa, that become more specific as one moves down the hierarchy. The largest and most inclusive category is the kingdom, which contains a vast number of species that share only the most basic similarities. In contrast, the category of species is the smallest and most exclusive, consisting of individual organisms that are similar enough to reproduce and produce fertile offspring together.
Closely related species are grouped into a genus, which is then followed by the categories of family, order, class, phylum, kingdom, and ultimately domains, each containing a larger number of species than the last.