So this question requires us to make an inference about what the author would agree with based on what she's written in the passage. The author, in the first paragraph, says that cohesive groups are more likely to discuss a wider range of ideas as group members aren't afraid to speak their minds. (B) is a direct application of this idea: unless groups are decently cohesive, it'll be hard for them to examine all relevant options critically, as individual members will be afraid, play it safe, and comply out of fear/recrimination. If you prefer diagramming, we can diagram (B) (recap that "X unless Y" can be diagrammed as ~Y - >X)
Not fairly high degree of cohesiveness - > difficult for a group to examine all relevant options critically in reaching decisions
Or, if it makes more sense to you, we can look at the contrapositive
Not difficult for a group to examine all relevant options critically in reaching decision - > fairly high degree of cohesiveness
Whether we diagram or not, or look at the first or second diagrammed version, we see that this is basically what the first paragraph tells us, and so we pick (B). Hope this helps!