Final answer:
Option 1), showing that premating isolation is the strongest in sympatry with no postzygotic isolation, would provide evidence of reinforcement between two species. It indicates that reproductive barriers have been reinforced when two species inhabit the same area to prevent the formation of less-fit hybrids.
Step-by-step explanation:
The evidence that would provide proof for reinforcement between two species is option 1), where premating isolation is strongest in sympatry; no postzygotic isolation in any cross. Reinforcement is a process through which natural selection increases reproductive isolation. It typically occurs when hybrid offspring have lower fitness compared with offspring from within-species matings, thus favoring reproductive barriers.
In this scenario, prezygotic isolation (barriers to mating or fertilization occurring before the formation of a zygote) is strongest in sympatry-- meaning it's more pronounced when two species coexist in the same geographic area, which suggests that these barriers have been reinforced to prevent the formation of less-fit hybrids. The absence of postzygotic barriers (barriers after zygote formation such as reduced hybrid viability or fertility) suggests that hybrids are not common, possibly due to strong prezygotic barriers already in place.