Final answer:
Intergroup competition increases group cohesiveness by uniting members toward a shared goal, while open discussions and external perspectives can prevent groupthink and manage group polarization.
Step-by-step explanation:
The factor that tends to increase group cohesiveness among the options provided is intergroup competition. Cohesiveness in a group is strengthened through a shared sense of purpose and common goals. When a group experiences competition with other groups, it often leads to members uniting and working cooperatively to achieve their objectives, thereby enhancing the cohesiveness within the group.
Groupthink may occur in highly cohesive groups where the desire for harmony or conformity in the group results in irrational or dysfunctional decision-making outcomes. To prevent groupthink, it is essential to encourage open discussion, acceptance of differing opinions, and periodic injection of external perspectives. Meanwhile, phenomena such as group polarization can amplify the initial attitudes of group members after discussion, which further highlights the importance of diverse viewpoints.