The conjunction while can be used to indicate contrast, but it can also be used to mean that two actions are parallel in time; thus, it is somewhat ambiguous as used in the original sentence. In addition, the non-underlined portion of the sentence sets up a comparison between the percentage of American carbon dioxide emissions that come from cars driven in the U.S. and the percentage of Japanese carbon dioxide emissions that come from cars driven in Japan. In the underlined portion of the original sentence, however, this comparison is lost because the phrase just over 10 percent of carbon dioxide does not clearly refer to the total amount of carbon dioxide emitted in Japan. Thus, you can eliminate Choice A. Since there is no 2/3 split, go through your remaining answer choices aiming to make your comparison parallel and logical. In addition, be on the lookout for a conjunction that is clearer than while.
In Choice B, the conjunction whereas clearly indicates a contrast, and the comparison mentioned above is parallel and logical, so keep choice B.
You can eliminate Choice C because the phrase compared to Japan renders the comparison set up in the non-underlined portion illogical and nonparallel. In addition, what the automobiles release is not specified. Choice D starts off well with the proper conjunction whereas, but the phrase just over 10 percent of the carbon dioxide emitted does not clearly refer to the total amount of carbon dioxide emitted in Japan. Thus, eliminate Choice D. Last, you can eliminate Choice E because the preposition with is an illogical connecting word and the phrase with the carbon dioxide from automobiles in Japan lacks a verb to parallel comes from in the non-underlined portion of the sentence, rendering the sentence's comparison nonparallel and illogical. In addition, the dependent clause where it is just over 10 percent does not clearly refer to the total amount of carbon dioxide emitted in Japan, so this clause does nothing to clarify the sentence's comparison. Furthermore, the pronoun it is ambiguous. Not three but four strikes against Choice E! That leaves you with Choice B, the correct answer.