Which of the following is NOT a prezygotic mechanism?

Hybrid inviability is indeed not a prezygotic mechanism; it is a postzygotic mechanism. Prezygotic mechanisms occur before fertilization and prevent mating or fertilization between species. These include temporal isolation, where species breed at different times, gametic isolation, which prevents fertilization when gametes from different species meet, and behavioral isolation, which involves differences in mating rituals or behaviors that prevent interbreeding. Since hybrid inviability occurs after fertilization, leading to the death of hybrid embryos or the inability of hybrids to survive to maturity, it is categorized as a postzygotic barrier, distinguishing it clearly from the other options, which are all strategies to prevent mating or fertilization from happening in the first place.