IMO D, I think it's a simple question

Fact: Those who think with a hierarchical mentality strive for situations in which their side is dominant and the other side is submissive
Argument: achieving parity of nuclear weaponry between the East and the West is not enough for Western military generals

--> 2 different scopes mentioned in fact & argument. To make an assumption for the argument, create a link between the 2 isolated piece of necessary information. Therefore, D is the best

Those who think with a hierarchical mentality strive for situations in which their side is dominant and the other side is submissive. In contrast, communal thinkers strive for parity among all sides. Therefore, achieving parity of nuclear weaponry between the East and the West is not enough for Western military generals.
Which one of the following assumptions would provide the most support for the conclusion above?
(A) Western military generals do not have the same mentality as do Eastern military generals -->mentality is irrelevant
(B) Parity in nuclear weaponry requires that military generals from both the East and the West think in communal terms -->no influence
(C) Western military generals want parity with respect to strength in nuclear weaponry between the East and the West -->no influence
(D) Western military generals’ thinking about relative strength in nuclear weaponry is hierarchical -->the best
(E) The thinking of military generals with respect to relative strength in nuclear weaponry is either hierarchical or communal --> this is not correct because it's not focus on the main point of the argument: why achieving parity of nuclear weaponry between the East and the West is not enough for Western military generals ?. Therefore, it's not a good base for the point to survive

As the above said, it's D. I would recommend going to powerscore's logical reasoning bible as they cover assumption questions in quite great detail.

yeah D