Concept:The biological species concept states that a species is a group of organisms able to interbreed and produce fertile offspring.
Explanation:Members of the same species are closely related and share similar genetic material.
They can mate naturally with one another and produce viable offspring.
Their offspring are also fertile, so they can continue reproducing successfully.
Higher ranks, such as kingdom, class, or family, contain many different species that do not usually interbreed successfully.
Therefore, the definition given in the question refers to the species level of classification.
Answer:D. species