This best captures the purpose, which is to support the claim that “a cohesive group—one whose members generally agree with one another and support one another's judgments—can do a much better job at decision making than it could if it were noncohesive.”

The topic of groupthink hasn’t been brought up or even alluded to when low group cohesiveness is mentioned. In addition, later in the passage the author never compares the features of groupthink to what occurs in low-cohesion groups.

The author never indicates that there are ways to make groups more open to dissent. Although we, as readers, might infer ways to encourage dissent, the author never chooses to present potential ways to encourage dissent. So (C) can’t be the purpose of the line.

The topic of groupthink hasn’t been brought up or even alluded to when low group cohesiveness is mentioned. In addition, the author never suggests that low-cohesion can result in groupthink.

The author never makes a proposal for overcoming low-cohesion. She never suggests that anyone should do anything to avoid low-cohesion. She simply describes the effects of low-cohesion. Although the author mentions what someone would need to do to “overcome” the fear of recrimination, this isn’t part of a “proposal” for overcoming the effects of low cohesion. Consider this analogy. “In order to get into medical school, you have to take the MCAT.” Am I proposing that you take the MCAT? No. I’m simply describing what you have to do to get into medical school. Similarly, the author isn’t proposing that anyone actually do something to overcome low cohesion. She’s simply describing what someone would have to do to overcome low cohesion.