Final answer:
The researcher can conclude that Sunshine periwinkle and Summer periwinkle are not pre-zygotically isolated from each other.
Step-by-step explanation:
The researcher can conclude that Sunshine periwinkle and Summer periwinkle are not pre-zygotically isolated from each other. This is because the fact that Summer periwinkle's blooming time can be manipulated suggests it should not be regarded as a species distinct from Sunshine periwinkle. Therefore, option 5) These plants are not pre-zygotically isolated from one another is the most appropriate conclusion.