The correct option is C Species
The branch of biology which deals with the identification and naming of organisms or in simple words in the classification of organisms is known as taxonomy. Biological classification is arranging the organisms into various categories on the basis of their structure, development and evolutionary history and the basic and the smallest unit of this biological classification is species.
Family, class, and genus come in a higher order of hierarchy in taxonomic classification hence, they cannot be the smallest and the basic unit of classification. Species are the smallest unit of classification.