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.