What describes a post-zygotic mechanism?

A post-zygotic mechanism refers to reproductive barriers that occur after fertilization has taken place. Specifically, these mechanisms work to prevent zygotes from developing into viable, fertile offspring. This could involve scenarios such as hybrid inviability, where the hybrid organism does not survive to maturity, or hybrid sterility, where the resulting hybrid is sterile and unable to reproduce. By addressing what occurs after the initial fusion of gametes, this concept illustrates how species can remain distinct even if they manage to mate and create a zygote.

The other options focus on barriers that occur before or during fertilization, rather than after, which places them outside the definition of post-zygotic mechanisms. For example, preventing gamete fusion and acting as a mating barrier pertain to pre-zygotic mechanisms, while aiding in fertilization does not refer to any barrier at all. Thus, the selected choice accurately captures the essence of post-zygotic mechanisms in reproductive isolation.