* Postzygotic reproductive barriers occur *after* the formation of a hybrid zygote (fertilized egg). These barriers prevent the hybrid offspring from developing or reproducing successfully.

* Hybrid sterility is a classic example of a postzygotic barrier. It means that even if a hybrid offspring is produced, it is unable to reproduce (like a mule, the offspring of a horse and donkey).

Let's look at the other options:

* B) Habitat isolation: This is a prezygotic barrier. It means that species live in different habitats and therefore don't encounter each other for mating.

* C) Gamete isolation: This is also a prezygotic barrier. It means that the eggs and sperm of different species are incompatible and cannot fuse.

In summary: Hybrid sterility is the only option that describes a reproductive barrier that occurs *after* fertilization, making it a postzygotic barrier.