In 1852 Robert Angus Smith published a detailed report of the chemistry of rain in a large area around the city of Manchester, England, noting
that the closer one came to town, the more the city air would become increasingly acidic.
The use of the idiom is correctly explained in the
Magoosh blog
Suppose A and B are two items or qualities or quantities, and we want to express how one of them changes as a result of the other one changing; that is, we want to express the interrelated nature of their changes. This is the formal structure of the idiom:
“the” (comparative adjective or adverb) (independent clause about A), “the” (comparative adjective or adverb) (independent clause about B)
Eg. The hotter the surface temperature of a star, the more light per square meter it radiates.
The higher they fly, the harder they fall.
Option Elimination -
(A) that the closer one came to town, the more the city air would become increasingly acidic - This sentence has two issues
1. The use of "would become" is wrong. The use of this subjective is for COUNTERFACTUAL/ HYPOTHETICAL or DOUBTFUL possibilities. Such as, "If I were you, I would explain the situation to her immediately." - COUNTERFACTUAL/ HYPOTHETICAL
or DOUBTFUL POSSIBILITIES - such as, "If I were to win a lottery, I finally would buy a car." Here, it's not Hypothetical but doubtful.
In this sentence, the air acidity increased in the past. It's neither COUNTERFACTUAL/ HYPOTHETICAL or DOUBTFUL.
2. More and increasingly are redundant. Both mean the same.
(B) that the city air became increasingly acidic the closer one came to town - Correct to use "past form" for what happened in the past. "one" is used as an "impersonal" or "generic pronoun" often used to refer to an unspecified person. In this sentence, "one" is used to refer to any person or observer who approaches the town, indicating that the observation holds true for anyone who comes closer to the town. It helps create a more general or impersonal tone in the sentence.
(C) that coming closer to town, the city air became increasingly acidic - the "coming closer to the town" modifier modifies "the air." As if the air is coming closer to the town. Here, the sentence means that "one" came closer to the town and not "air."
(D) that the more the city air became increasingly acidic, the closer one was to town - the opposite relationship. As explained above in
Magoosh's example, this wrongly shows the second part as the result of 1st part.
(E) the city air becoming increasingly acidic as one would come closer to town - we need "that" after noting.