IMO A

All the superlative degrees (surest, easiest, and least expensive) are being highlighted/emphasized in this sentence. Knock out B,D and E.

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

(B) one of the most sure, easy, and least expensive ways to begin a halt or even reverse

(C) one of the surest, easiest, and least expensive ways that would begin halting or even reversing (That refers to "the widespread planting of trees" rather than explaning purpose of the widespread planting of trees i.e "to reverse the buildup of carbon dioxide in the air" . Moreover Manhattan SC Advance lesson has one chapter on V-A-N preference which tells you to select verb over adjective and noun--> reverse VS reversing)

(D) a most sure, easy, and inexpensive way beginning the halting and even reversing of

(E) the most sure, easiest, and inexpensive way that would begin halting or even reversing

naveen451 wrote: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.

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

(C) one of the surest, easiest, and least expensive ways that would begin halting or even reversing

OA IS A. WHY NOT C

C changes the meaning of sentence. Lets look at A, orginal sentence - it says 'planting of trees...would be the surest way to halt or reverse carbon buildup'. It means by planting of trees we would help to revese of carbon buildup (effect).

C says 'the ways would begin halting or reversing buildup'. It implies, the ways (all those mentioned) would act on its own to reverse.

IMO A