Which of the following is NOT a characteristic of primary groups?

Primary groups are typically characterized by close, personal relationships among their members, often involving strong emotional connections and a high degree of intimacy. These groups, such as families and close friends, are marked by enduring interactions that develop over time and are central to individuals' social lives.

The unique trait that sets primary groups apart is the informal and unstructured nature of their interactions. Members of primary groups engage in spontaneous, free-flowing communication, creating a dynamic environment where relationships can flourish without the constraints of formal rules or organizational hierarchies. In essence, primary groups prioritize emotional bonds and personal connections rather than systematic, formal structures typically found in secondary groups, which are more task-oriented and less personal.

Thus, the characteristic of having a formal structure and organization does not align with the fundamental nature of primary groups, making it the correct answer.