Explanation:

species

Each unit or category of classification is termed as a taxon. It represents a rank. 
The Swedish botanist Carl Linnaeus is regarded as the Father of taxonomy,  as he developed a system known as Linnaean taxonomy for categorisation of organisms and binomial nomenclature for naming organisms.
The term taxonomy was originally coined by Augustin Pyramus de Candolle in 1813. Linneaus is 'The Father of Plant Taxonomy'.
The basic unit or smallest taxon is the species. In biology, a species is the basic unit of classification and a taxonomic rank, as well as unit of biodiversity 
 So the correct option is Species.