Though the law will require emissions testing of all diesel vehicles, from tractor trailers to excursion buses, it will have no effect on sport utility vehicles, almost all of which are gasoline
powered, and will not be subjected to emissions-control standards as stringent as they are for diesel-powered vehicles.
Option Elimination -
A. powered, and will not be subjected to emissions-control standards as stringent as they are for - As GMATNinja beautifully explained the distinction between the usage of "they," the third-person plural pronoun, and "those," the plural demonstrative pronoun. "they" maintains continuity with a previously mentioned noun, while "those" introduces a new set for comparison or contrast.
The Smiths adopted two puppies last month. They have quickly become a beloved part of the family. Here, "they" refers to the same puppies.
The laptops she has are much more powerful than those I have. Here, "those" refers to the "laptops" but a different one. While we are on the same topic, "these" is another demonstrative plural pronoun used for plural things closer to the speaker. These cookies are delicious. And "those" refers to plural things farther from the speaker. E.g., Those birds are flying high in the sky.
Here, "those" refers to the emission-control standards but not the same standards for the diesel-powered ones. Diesel-powered uses diesel, and gasoline-powered uses refined fuel, which is gasoline. Moreover, Diesel powered have a higher compression ratio than gasoline, which means their operating principle differs. Can we use the same standards for both? No.
B. powered, and therefore not subjected to emissions-control standards that are as stringent as those of - "those for" is better as "diesel-powered" vehicles don't have standards on their own. We are talking about "standards" for "diesel-powered vehicles."Moreover, "subjected to" involves undergoing a process or experience, while "subject to" refers to under the influence or governance of certain conditions or rules.
E.g., The cars we drive are subjected to torture track testing during their development stage to ensure that all the parts are adequately designed.
All employees are subject to the company's code of conduct. Moreover, the "that are" relative clause is redundant.
C. powered and therefore not subject to emissions-control standards as stringent as those for - ok.
D. powered, which are not subject to emissions-control standards as stringent as they are for - "which" without "and" can refer to "gasoline-powered," which, because of the presence of the linking verb "are" is a subject complement of the earlier "which" but "gasoline-powered" is also adjectival and "which" as a pronoun" generally refers to noun and not adjectives. Moreover, we need "they" and not "those."
E. powered and therefore they are not subject to emissions-control standards as stringent as those of - "they are" adds unnecessary redundancy as we already have a subject and a verb that can refer to this part after "and." Moreover, we need "those for" as "diesel-powered" vehicles don't have standards on their own.