Genetic engineers may use cloning techniques to develop more productiv

Here's the official explanation provided by the GMAC for this question:

The sentence addresses the scope and limitations of cloning techniques. These techniques, we are told, can improve agricultural plants, but it is conceded that, contrary to what science-fiction writers have imagined, such techniques may never be successfully used to create a new strain of humans. The main flaw in the given sentence is the use of the unidiomatic despite that, where although or even though would better express the intended concession.

Option A: As explained, this version fails because of the use of despite that. Despite normally functions as a preposition governing a pronoun, noun, or noun phrase rather than a noun clause introduced by that. Treating despite that as a conjunction that can function like although and introduce a clause is not generally accepted as grammatically correct.

Option B: The forward-reference of the pronoun they is not sufficiently clear; there is a natural tendency to read it initially as referring to cloned human beings, even though that makes little sense in the context of the sentence as a whole.

Option C: The pronoun they is naturally taken as referring to science-fiction writers, but that reference makes little sense in the context of the sentence as a whole; it is unlikely that the intended thought is that science fiction writers would use cloning techniques. The relative clause that … create is a modifier of a race … human beings, but because the entire phrase (noun phrase plus the modifying relative clause) conveys what some fiction writers suggest, the noun phrase cloned human beings that genetic engineers may never create is incoherent (cloned by definition entails being genetically engineered).

Option D: Correct. The pronoun they clearly refers back to genetic engineers. The words even though correctly introduces a clause used to concede something.

Option E: The use of the passive verb form may be produced is unnecessarily convoluted and provides no improvement on the given sentence. The fact that they is preceded by several plurals tends to confuse the issue of which plural noun it refers to. Even if context suggests that it refers to genetic engineers, grammatically it should refer back to the subject of the main clause (more productive and disease-resistant plants).<a name="_GoBack"></a>

The correct answer is D.

Please note that I'm not the author of this explanation. I'm just posting it here since I believe it can help the community.