The type of insurer that is unincorporated, solely for the benefit of its members, and not for profit is known as?

The correct choice is a fraternal organization. This type of insurer is typically formed to provide insurance to its members who share a common bond or affiliation, such as a religious, ethnic, or occupational connection. Fraternal organizations are characterized by their non-profit nature and their primary goal of providing benefits to members rather than generating profits.

While mutual insurance companies also benefit policyholders and can be non-profit in concept, they are incorporated and may be larger in scope. Therefore, fraternal organizations stand out due to their unincorporated status and focus purely on member benefits.

In contrast, risk retention groups are formed specifically to provide liability insurance to their members, particularly in industries that face high risks, while stock companies are for-profit entities owned by shareholders. These distinctions clarify why fraternal organizations are the correct answer in this context.