Question

the biological species concept defines a species on the basis of:
the ability to mate and produce fertile offspring.
similar physical appearance.
recognizing one anothers mating behaviors.
close genetic relationships.
similar levels of genetic diversity.

Explanation:

The biological species concept, proposed by Ernst Mayr, defines a species as a group of organisms that can interbreed in nature to produce fertile offspring, which is the core criterion distinguishing it from other species concepts (e.g., morphological species concept focuses on physical appearance).

Answer:

the ability to mate and produce fertile offspring.