The disproportionately high appearance of polydactylism among the amish suggests that a. it is a common mutation b. it is a common birth defect in pennsylvania c. it was overrepresented in the gene pool of the original amish settlers d. humans are prone to this birth defect

The answer is c. it was overrepresented in the gene pool of the original Amish settlers.

A founder effect is a result of geographical separation of a few individuals from the original population. Those few founding individuals will form a new population in which will be present alleles of the few founding individuals. It is probably that in founding individuals, in this example the original Amish settlers, polydactylism was overpresented, and since there was no gene exchanging with other population, polydactylism remained among Amish.