Answer:
Postzygotic reproductive barriers
Explanation:
Post-zygotic reproductive barriers are the adaptations developed in the organism which occur after the zygote (fertilization product) has been formed.
The Post-zygotic barrier involves the hybrid inviability which refers to the reduction on the viability of the offspring and hybrid sterility or that is the production of the fertile reproductive barriers.
The production of mule by the mating of horse and donkey results in the infertile mule which is considered the mechanism of the post-zygotic barrier.
Thus, Postzygotic reproductive barriers are the correct answer.
