While studying the genetic makeup of corn, a new class of mutant genes was discovered by Barbara McClintock, a discovery which led to greater understanding of cell differentiation.
(A). a new class of mutant genes was discovered by Barbara McClintock, a discovery which led
(B). a new class of mutant genes in corn were discovered by Barbara McClintock, leading
(C). Barbara McClintock discovered a new class of mutant genes, and it led
(D). Barbara McClintock discovered a new class of mutant genes, a discovery that led
(E). Barbara McClintock, who discovered a new class of mutant genes, leading
考察同位语结构
Studying放开头，要找对应的逻辑主语，所以a new class无法发出study只有CDE
E中很明显没有谓语
C中it 无法指代一件事情
D中SVO,N+that是抽象性同位语结构，可以指代一件事情
所以选择D