Final answer:
Reinforcement in speciation mainly involves strengthening of the preexisting prezygotic isolation mechanisms, which prevent species from interbreeding.
Step-by-step explanation:
The process of reinforcement in terms of speciation involves strengthening prezygotic barriers to reproduction among species. This takes the form of behaviors, temporal isolation, or other mechanisms preventing different species from mating. Therefore, when considering the options provided, preexisting prezygotic isolation (option 3) is required for the process of reinforcement. Preexisting postzygotic barriers (option 2) may also contribute to speciation by reducing the fitness of hybrids, but they come into play after fertilization.
While genetic variation (option 1) is critical for many processes in evolution, it is not specifically required for reinforcement, which is more about the isolation between species rather than variations within a species. The correct answer is that reinforcement requires preexisting prezygotic isolation.