Confused on why A is correct

Hi Sallyanne, The sociologists disagree with the idea that such a complex thing as the rise of an organization can be caused by a simple phenomenon. Since they dispute the idea that the rise of an organization can be caused by economically motivated voting on these grounds, they must believe that economically motivated voting is a simple phenomenon- or in other words, that it is not complex.

Why would B be incorrect? Is it just too broad?

It is indeed. While we have an example of B here, we don't know that b is generally true.