Answer: D. Species.
Step-by-step explanation:
The biological species concept is usually described as the largest group of organisms in which any two individuals of the proper mating varieties can generate fertile offspring, often by sexual reproduction. It refers to the classification containing related organisms that have common features and are capable of interbreeding.
A species, in biology, is the primary unit of classification as well as a taxonomic order of organisms. Furthermore, is a unit of biodiversity.