First things first, this is a Strengthen question, so we need some new information in the answer choice to help strengthen the author's contentions. The relevant contention is that groupthink is more likely to happen in highly cohesive groups (see the second paragraph for support for this claim). A cohesive group, so we have our definitions straight, is a group where members generally agree and support each other's judgments (see lines 1-3). What does (C) tell us? (C) tells us that in many groups with members who generally distrust each other's judgments, there were no cases of groupthink. These kinds of groups are groups with low cohesiveness, and so groups that don't have as much cohesiveness don't have groupthink. This supports the author's argument, since the author is arguing that high cohesiveness may cause groupthink. The author discusses cases in which high cohesiveness is present, and groupthink is also present. (C) shows that in cases where high cohesiveness isn't present, groupthink isn't present. In other words, once you remove the author's (alleged) cause, the effect goes away as well, and this helps prove the author's point. Hope this helps!