Final answer:
The best way to reduce prejudice between hostile groups is through cooperative interaction, focusing on common goals, and diversity training that facilitates intergroup contact and understanding.
Step-by-step explanation:
If two groups are hostile to each other, one of the best ways to reduce the level of prejudice between them is to engage them in activities that promote cooperative interaction toward achieving common goals. Studies, such as the Robbers Cave experiment and the jigsaw classroom technique, have shown that when individuals from opposing groups work together on a task that needs all members to participate, the level of prejudice tends to decrease. This cooperative effort can be supported by diversity training in the workplace and other activities that foster intergroup contact and collaboration. In addition to these strategies, recognizing and addressing internal biases, expressing empathy, and acknowledging past sufferings can further facilitate reconciliation and reduce aggression between groups.