The correct option is C Classes
Taxon (plural: taxa) represents taxonomic group of any rank. Each plant belongs to a hierarchy of taxa which are assigned taxonomic ranks. The seven essential ranks of taxa in descending order are kingdom, division/phylum, class, order, family, tribe, genus and species. Order represents group of families that share a set of common characters; option B is incorrect. A group of related order constitutes a class; option A is incorrect. A group of classes that share some common characters constitute a division/phylum; option C is correct. A group of related tribes/ sub families constitutes a family; option D is incorrect. Thus, the correct answer is option C.