Class, Order and Species are examples of taxonomic category or rank. Taxonomic rank shows the level of a taxon. Taxonomic ranks are useful for comparing two taxa ( plural of taxon ).
The taxon is the term for any taxonomic category. It is used to denote any taxonomic unit. Taxon can be simply defined as the group of organisms occupying a particular category/rank. For example aves, mammalia are examples of taxon and 'class' is their taxonomic rank. Similarly, primata is an example of another taxon and 'order' denotes it's taxonomic rank.
So, the correct option is 'Taxon'.