Answer: D)When they can interbreed to produce fertile offspring.
Step-by-step explanation:
A species can be define as the group of organisms that exhibit common physical and physiological characteristics. The members of such group are capable of inbreeding among themselves and producing viable fertile offsprings.
On the basis of the above description, this can be said that two animals can be classified as belonging to the same species when they can interbreed to produce fertile offspring.