How is a free net characterized?

A free net is characterized by a "first come, first serve access" mechanism. This means that any user can access the network and transmit messages as long as they are willing to wait their turn. The concept emphasizes equal opportunity for all users to access the network resources without preferential treatment or restrictions based on the message content or sender's identity.

In contrast, other options describe characteristics that do not align with the principles of a free net. Formal messages only or high-priority messages imply a level of monitoring and control that restricts access and prioritization. Completely unmonitored communication might suggest a chaotic communication environment that could lead to misuse, which does not align with the orderly nature of a first-come, first-served approach. Thus, the essence of a free net lies in its open and egalitarian access system, which is well captured by the idea of first come, first serve.