We should be comparing the rate of mainstream American businesses and rate for businesses in the Amish community so answer should be D.
“Unlike mainstream American businesses, more than half of which fail, the failure rate for businesses in the Amish community hovers around 5 percent. A. Unlike mainstream American businesses, more than half of which fail B. Unlike mainstream American businesses, in which the failure rate is more than half C. Unlike mainstream American businesses, where more than half of them fail D. While the rate of mainstream American businesses failing is more than half E. While more than half of mainstream American businesses fail”
you are doing such a comparison with “unlike”. No such requirement is mandated with a subordinate conjunction such as “while”.
D – “the rate of American businesses + [Modifier = failing]” –> this does not make any sense.
