When I read this question, my eye went to the "declin-e/ing" "OF" idiom too, but I usually tell my students to leave idiom splits for last -- in general, unless you are 99.9% sure of an idiom (a good test is: would you bet someone 500 bucks you're right?)--then it is safer to eliminate choices with other concrete grammatical errors first.
The next concrete split I noticed in the answers is the use of "including" and "included" before "among" in choices A and C, respectively. The usage of either of these words makes the usage of "among" redundant, so both choices are out. This left me with B, D, and E.
Choice B starts with the relative pronoun "which," which introduces a noun modifier. Noun modifiers must touch the nouns they modify. Since we want to modify "the challenges" rather than "the company," choice B is out.
To choose between D and E, you should know that the GMAT generally does not allow usage of demonstrative pronouns (this, that these, & those) where nouns are required-- additionally, "among" is a preposition and therefore should be followed by the object case. So you can't say "among these" and must say "among them" instead. Even without depending on the idiom distinction, you can arrive at the answer: D.