Final answer:
The least inclusive category in the biological hierarchical system is the species, as it is the most specific and includes organisms that can interbreed to produce fertile offspring.
Step-by-step explanation:
In the biologic hierarchical system of organization, the least inclusive category is the species. This system is often referred to as the Linnaean system of classification, after Carl Linnaeus. It is structured as a hierarchy, starting from the most inclusive categories such as domain and kingdom, and moving down to more specific ones like phylum, class, order, family, genus, and ultimately species. Among the options given - phylum (A), order (B), kingdom (C), and species (D) - the species is at the bottom of the hierarchical structure and is therefore the most specific and least inclusive group.
To illustrate, the kingdom is a broad grouping with organisms sharing just a few similarities. As the hierarchy narrows, more specific characteristics define the organisms in each category, ending in the species category, which includes organisms similar enough to interbreed and produce fertile offspring. Therefore, the correct choice here, which represents the least inclusive category, is D) species.