Because natural gas is composed mostly of methane, a simple hydrocarbo

First, you're right, the juiciest decision point is between "less" and "fewer." As is usually the case, the choice comes down to context and meaning. If we're talking about the number of pollutants emitted, we'd use "fewer" as this would be countable. If we're talking about the quantity of pollutants, we'd use "less" as the quantity of a volume isn't really countable. (A good rule of thumb here is whether the item in question can be expressed as an integer value. We can count to 3. But it doesn't make sense to count to 3.7.) Much more on countable vs. non-countable modifiers in this video.

Here it seems more logical to talk about the quantity of "certain pollutants." If one vehicle emitted four kinds of pollutants and another three kinds, it's not clear if we should care. It would really depend on how much was emitted. Also, if we were talking about the number of pollutants, it would be far simpler to just writer "fewer pollutants," as opposed to "fewer of certain pollutants." Because we'd prefer "less," to indicate a non-countable quantity, (B) and (D) are out.

Next, take another look at (A):


Now it sounds as though we're comparing the amount of pollutants emitted by vehicles powered by natural gas to the amount of pollutants emitted by the burning of gas or diesel in general. That's an awfully wonky comparison. Shouldn't we be comparing emissions from vehicles running on natural gas to emissions from other types of vehicles? I'm tempted to get rid of (A), but first, I might see if a more logical comparison is available.

(C) gives us the following:


If anything, this one is worse. Now we're comparing emissions of vehicles powered by natural gas to gasoline or diesel fuel themselves. Fuel, so far as I know, doesn't just spew out emissions on its own. You have to use or burn it. So (C) is illogical and out of contention.

And here's (E):


Now we're comparing vehicles powered by natural gas to those (vehicles) burning gasoline or diesel. The vehicle to vehicle comparison is more logical than the comparison we get in (A) between vehicles powered by natural gas and the general burning of gas and diesel. Because (E) uses "less" correctly and offers the most logical comparison, it's our winner.


It's mostly a question of whether we need the helping verb, "do" or "does" to clear up an ambiguity.

Consider an example:

Tim hates Pokemon more than his wife.

Here, for the sake of Tim's marriage, we really want a helping verb. The reason is that the sentence, as is, has two possible interpretations:

1) Tim hates Pokemon more than his wife hates Pokemon.
2) Tim hates Pokemon more than he hates his wife. (That's a problem, both for Tim and for the GMAT.)

However, if we write, "Tim hates Pokemon more than his wife does," it's now clear that the first interpretation is the correct one.

But not every sentence has two reasonable interpretations. For example, "Tim runs faster than Ben," doesn't include the helping verb "does," but because there's only one coherent way to understand the sentence, it's fine as is.

(E) is more like the second scenario that has only one logical interpretation. In other words, no reasonable person would read "Vehicles powered by natural gas emit less of certain pollutants than those burning gasoline or diesel fuel," and think there was any chance that the sentence meant that vehicles powered by natural gas emit less of certain pollutants than they emit vehicles burning gasoline.

Because there's no ambiguity here, the helping verb isn't required. And there's no rule about whether "does" should come before the subject or after.

I hope that helps!