感谢楼主的耐心解答。这样一来更加清晰明朗了。因为如果but是介词,c选项里,sb argued that sth is not...products of natural selection, done, but(prep)n....那么这里的by-products实际上成为了natural selection的同位语。(把插入语favored去掉)
Paleontologist Stephen Jay Gould has argued that many biological traits are not the products of natural selection, favored because they enhance reproduction or survival, but are simply random by-products of other evolutionary developments.