We should notice right away that "while studying the genetic makeup of corn" is a modifier, which means it must describe the closest noun. In A and B, the closest noun is " a new class of mutant genes", while in C, D, and E, the closest noun is "Barbara McClintock". It doesn't make sense that a class of genes would study anything, so we can eliminate A and B. It does make sense that a person (like Barbara McClintock) would study corn's genetic makeup, so we'll keep C, D, and E. We'll look at each individually
C. While studying the genetic makeup of corn, Barbara McClintock discovered a new class of mutant genes, and it led to greater understanding of cell differentiation.
If we break the sentence apart at the comma + "and", we end up with two complete sentences, so no sentence structure issues here. However, we should notice that we have a pronoun in the second half of the sentence that we don't have in either D or E: "it". What does "it" refer to? The only subject "it" could refer to is "a new class of mutant genes". So the sentence indicates that a class of genes led to greater understanding of cell differentiation.
Looking back at the original sentence, we see that this isn't the intended meaning of the sentence. The original sentence tells us that the discovery of this new class of genes was what led to greater understanding of cell differentiation. The word "discovery" isn't in answer choice C, so there's no way for "it" to refer to "discovery". So answer choice C doesn't match the intended meaning of the sentence. Eliminate.
D. While studying the genetic makeup of corn, Barbara McClintock discovered a new class of mutant genes, a discovery that led to greater understanding of cell differentiation.
No obvious errors in this sentence. We'll hang onto it.
E. While studying the genetic makeup of corn, Barbara McClintock, who discovered a new class of mutant genes, leading to greater understanding of cell differentiation.
We notice that this sentence adds another modifier: "who discovered a new class of mutant genes" describes Barbara McClintock. When we have a bunch of modifiers in a sentence, it's a good idea to eliminate all of them to see if we're still left with a sentence that makes sense. Modifiers should not be grammatically necessary to the grammar of the sentence, so we should still have a complete, logical sentence without them:
While studying the genetic makeup of corn, Barbara McClintock, who discovered a new class of mutant genes, leading to greater understanding of cell differentiation.
Barbara McClintock leading to greater understanding of cell differentiation.
When we pull all of the modifiers out of the sentence, we're left with a fragment. This means that the sentence was a fragment the whole time, and we can eliminate E for sentence structure issues.
This leaves us with D as the correct answer.
