B --> "were" is incorrect for singular "a new class"
C --> "it" is ambiguous. It raises a question whether genes led to... or the discovery led to...
D --> This clearly specifies that it was discovery that led.
E --> Usage of commas is not justified. It confuses what is leading. Is it Barbara or discovery.
One more point to note in this question.
In the initial sentence "a new class" is coming after comma, suggesting that mutant genes was studying genetic makeup












