The correct answer is C. If mules could produce offspring
Step-by-step explanation:
In biology, a species is defined as a group of organism that shares common features and produces fertile offspring through sexual reproduction which usually implies a female and male organism of the same species. This means organisms are classified into species not only because they share certain features including DNA, behavior or morphology (the form of the organisms), but because they are able to reproduce and its offspring is fertile.
Considering this in the case of donkeys and horses they are classified in different species, because even though donkeys and horses share common features including similarities in the morphology, behavior and even DNA, the offspring of these species which is called mules is not fertile (sterile) and therefore they do not belong to the same species. Thus, the discovery that would define horses and donkeys as the same species is "if mules could produce offspring", because only organisms that belong to the same species are able to produce fertile offspring.