The "illusion of invulnerability, which creates excessive optimism" is where I found support for B as well. It basically says that the members of the group believe the group to be a perfect decision-making entity. Still, I found "strong belief" to be too strong at first, which forced me to consider the other answer choices well.
A. They keep trying to trick us into thinking that adversaries have something to do with this passage. We're not going to let them. Incorrect.
C. The "high stress" idea is another one they keep trying to push. There is no support for this in the passage. Incorrect.
D. I remember nothing about cliches or vapid arguments. Fortunately, we can check this very easily, because the components of group think are neatly contained in the last paragraph.
E. This would be the solution to groupthink, not a symptom of it.
After reviewing the other answer choices, my complaint about answer choice B seems much smaller. This is a compromise we have to make sometimes in reading comprehension.
