Explanation

Creating a filter: the original sentence has a defect. To match the rest of the sentence, the phrase "treating single gene defects" should be in the form of a noun. A noun, in its place, will be unambiguous in describing what gene doping is. As written, the phrase potentially describes what gene doping is doing, as if Mr. Gene Doping were leaning over a table, treating single gene defects. So we eliminate (A) and can filter the answer choices by looking for a noun phrase to replace "treating single gene defects."

Applying the filter: our filter points us to choices (B) and (E). Choice (E), however, expresses something other than the intended meaning; it says that gene doping is the single gene defects. So (E) is out. (C) is not grammatical; it includes a verb that has no subject. The infinitive in (D) is meant to convey purpose or intention, but the function of the phrase is as an appositive that modifies gene doping, and such an appositive is simply an equal sign, grammatically speaking. Gene doping equals the treatment of single gene defects. The correct answer is (B).