It may also be helpful to note that, in general, the test-writers don't like indicative pronouns (these, those, this, that, etc) that float free without attached nouns.
For example...
I'm not sure how I feel about that rule. (
Correct)
I'm not sure how I feel about that. (
Incorrect)
That knocks off C and E.
Two of the choices that are left-- A and D, include
object pronouns ("them") while B contains no pronoun at all. It might be temping to nix all pronouns, but as daagh mentioned above, ", which" must touch the noun it is modifying, and in choice C it is touching "company"--eliminate B.
But, you may ask, what about the fact that there are *several* plural nouns earlier in the sentence ("stockholders, "investors," and "challenges")? Doesn't that mean the pronoun is ambiguous?
Unfortunately, no. A degree of pronoun ambiguity can be tolerated if it is grammatically clear from the sentence which of the possible antecedents is intended (notice, this is different from being clear because "I can tell what they probably meant from the content")--grammatically clear means clear from the *structure*)
(1) "stockholders" modifies "meeting" and is not a standalone noun-- out
(2) "investors" is the subject of the clause ("INVESTORS HEARD"), but we want an object noun (because we have THEM), not a subject noun. If our pronoun was "they" we would be referring to "investors"--out
(3) "challenges" is the object of the preposition ("presentation ON THE NUMEROUS CHALLENGES") -- here's our guy!
One last clue-- parallelism is very powerful on the GMAT, and the structure in D is nicely parallel:
(D) ...among them the THREAT OF....and the DECLINE OF... (yay)
...whereas in A we have
(S)...among them the THREAT FROM...and THE DECLINING SALES OF (less yay)
That's more ammo for (D) being the correct response.