The correct option is A Species
Species constitute a group of pheontypically similar organisms with common ancestory that can interbreed freely to produce fertile progeny; option A is correct. A genus is a taxonomic rank that constitutes single species of monophyletic group of species that share a sum total of characters, though they cannot interbreed which makes option B incorrect. A group of closely related genera, that share some common morphological features but are reproductively incompatible, constitutes a family; option C is incorrect. Taxon represents any level of grouping; it is basic unit of classification. Species, genus, family are different taxon which are not necessarily capable of interbreeding, option D is incorrect. Thus, the correct answer is option A.