Final answer:
The option which is not an example of a reproductive barrier is 'Walrus harems with dominant males and non-harem males' because it describes intraspecific behavior, not a barrier between species.
Step-by-step explanation:
The question asks to identify which option is not an example of a reproductive barrier. Reproductive barriers are mechanisms that prevent species from mating with each other or ensure that if they do mate, they do not produce fertile offspring. These can include behavioral differences, temporal isolation, mechanical incompatibility, and gametic barriers, among others.
Option A, 'Walrus harems with dominant males and non-harem males', is representative of a behavioral adaptation within a species rather than a barrier between species. Options B, 'Cicadas with different mating cycles', and C, 'Horses and donkeys producing mules', are examples of temporal isolation (reproductive cycles do not coincide) and postzygotic barriers (offspring is sterile), respectively. Option D, 'Elaborate courtship dance in species A and vibrant plumage in species B', represents distinct courtship behaviors, which are prezygotic barriers.
Thus, the option that is not a reproductive barrier is A, as it describes behavior within a species rather than between species.