This passage is considered pretty tough by most people's standards, so I'm surprised that any question fell through the cracks.

I think your confusion came from line 26, which says that in groupthink groups, there is no danger that "individuals will conceal objections they harbor regarding a proposal favored by the majority", which would seem to support (E).
However, the rest of that sentence goes on to say the reason objections to the majority won't be concealed is because objections to the majority won't be considered in the first place.
Lines 25-31, especially "they will think the proposal is a good one without attempting to carry out a critical scrutiny that could reveal grounds for strong objections", seem to support (B) and oppose (E).
Also, lines 47-48 contradict (E).
The reason groupthink is a problem is because people buy in to whatever the majority is thinking, no longer raising/considering/humoring possible objections.
Hope this helps.
=== other answers ===
A) there wouldn't be adversaries within a group that is vulnerable to groupthink - groupthink only occurs within tight-knit, highly cohesive groups.
C) high stress was never mentioned as a component of groupthink
D) this was mentioned in paragraph 1 in reference to how timid members of a low cohesion group might behave