Pollutants in itself is not countable for sure. Don't you think certain pollutants are countable.
This Q can be answered without worrying about the less v few issue. If you look at the comparisons, only E stands good. vehicles compared to vehicles
For a moment I thought D also compares vehicles to vehicles. But D compares vehicles a noun to the verb does. Hence wrong.
We are not counting the pollutants, we are referring to the quantity/amount of X number of pollutants emitted by the NG and D/F. Quantity and amount are uncountable, hence few and E