Many environmentalists believe that the widespread planting of trees,

Hello NamanPro,

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To answer your query, the use of "ways" in the original sentence is acceptable because the sentence conveys the meaning that the "widespread planting of trees" is one way among many different ways to begin to halt or even to reverse the build-up of carbon dioxide.

That having been said, the use of "ways" is not entirely necessary; the use of the phrase "a...way" and "the...way" in Options D and E respectively also convey logical meanings, albeit one's different from the original meaning.

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This question is designed to test the parallelism in a sentence
1) We can make a 3/ 2 split with respect to parallelism of the "surest, easiest, and least expensive" these 3 words should be parallel. Using this we can eliminate B ,D & E.
2) Among A & C the phrases "to halt" and "to reverse" are the verbs that should be parallel to believe which is the main Verb, which is not the case with C so eliminate this.

Correct answer would be A.

In option E, why is ???ways??? wrong? using ???along??? in the sentence and not ???and??? means that there is only 1 point, for which we should use singular. Also, why is ???most??? considered to be distributed on all the elements in the list in option E?

Many environmentalists believe that the widespread planting of trees, along with the conservation of existing forests, would be one of the surest, easiest, and least expensive ways to begin to halt or even to reverse the buildup of carbon dioxide in the air.

Option Elimination -

(A) one of the surest, easiest, and least expensive ways to begin to halt or even to reverse - ok

(B) one of the most sure, easy, and least expensive ways to begin a halt or even reverse - "most sure," "most easy," "most least" - not right. We need the surest, easiest, and least. "a halt" noun is not parallel with the "reverse" verb. We need "to halt" and "to reverse" for parallelism.

(C) one of the surest, easiest, and least expensive ways that would begin halting or even reversing - the relative clause "that would begin halting or even reversing" modifies ways and hypothetical meaning as if ways may or may not achieve the desired outcome.

(D) a most sure, easy, and inexpensive way beginning the halting and even reversing of - we need "the" before most. It's messed up. We need the surest, easiest, and least. We need "to begin."

(E) the most sure, easiest, and inexpensive way that would begin halting or even reversing - We need the surest, easiest, and least. Moreover, the relative clause "that would begin halting or even reversing" modifies ways and hypothetical meaning as if ways may or may not achieve the desired outcome.