Taxon is used for

The correct answer is "any rank of taxonomic Hierarchy".

Explanation:
In biological classification, the term "taxon" (plural: taxa) refers to a concrete taxonomic group of any rank or level in the hierarchical system of classification. It represents a real, tangible biological group (such as a specific species, genus, family, order, class, phylum, or kingdom) rather than just an abstract category.

Let us break this down step-by-step to understand why this option is correct:

• Taxonomic Hierarchy: Biological classification organizes living organisms into a nested framework of categories in descending order: Kingdom, Phylum (or Division), Class, Order, Family, Genus, and Species.
• Category vs. Taxon: A taxonomic category is an abstract rank or level (for example, "Species" or "Class"). In contrast, a taxon is the actual biological group occupying that rank. For instance, Homo sapiens is a taxon at the species level, Mammalia is a taxon at the class level, and Chordata is a taxon at the phylum level.
• Application to Any Rank: Because a taxon designates any level of grouping in this system, it is not restricted to specific ranks like genus and species, or ranks up to phylum. It is a general term applied to any taxonomic grouping at any rank of the taxonomic hierarchy.