For speciation to occur, the most critical factor among the options provided is c) reproductive isolation.
Reproductive isolation is a condition where different species may live in the same area but cannot produce offspring or their offspring are sterile. This separation can occur through various mechanisms such as temporal isolation, behavioral isolation, and mechanical isolation, among others.
While polyploidy (a), hybridization (b), and geographic isolation (d) can lead to speciation, they typically do so in conjunction with reproductive isolation. For example, polyploidy is common in plants and can create a reproductive barrier, but it alone does not ensure speciation without reproductive isolation. Hybridization can introduce genetic material between species but often does not lead to full speciation without the establishment of reproductive barriers. Geographic isolation can separate populations, but the populations must also develop reproductive isolation traits over time for speciation to happen.
In summary, while all the factors listed may contribute to the process of speciation in different contexts, reproductive isolation is the essential mechanism that ensures two populations evolve into distinct species.