Which of the following barriers occurs before the fusion of gametes?

The correct choice is pre-zygotic barriers. These barriers occur before the fusion of gametes, which means they prevent fertilization from taking place. Pre-zygotic barriers can include various mechanisms such as temporal isolation (different mating seasons), behavioral isolation (different mating rituals), mechanical isolation (incompatibility of reproductive structures), and gametic isolation (incompatibility of sperm and egg).

This type of barrier is crucial in maintaining species integrity by preventing different species from interbreeding, ensuring that each species remains genetically distinct. For instance, if two species occupy the same habitat but mate at different times of year, the temporal isolation serves as a pre-zygotic barrier.

The other options describe situations that occur after fertilization or relate to population dynamics rather than reproductive isolation before gamete fusion. Hybrid inviability and hybrid sterility occur post-zygotically, as they refer to the viability of hybrid offspring or their ability to reproduce, respectively. A population bottleneck refers to a significant reduction in population size and genetic diversity due to environmental events, not directly related to reproductive barriers. Therefore, pre-zygotic barriers accurately represent barriers that occur before the fusion of gametes.