The statement "a network administrator is usually not required on a network" is generally true for a peer-to-peer network (option a).
In a peer-to-peer network, each computer (or peer) has equal status, and there is no centralized server. Users on the network can share resources directly with each other, and there is less need for a dedicated network administrator.
In client/server networks (option b), intranets (option c), and metropolitan area networks (option d), there is typically a higher level of complexity, and network administrators are often required to manage and maintain the network infrastructure, security, and other aspects. These types of networks usually involve a centralized server or servers that control and provide resources to client devices.