The correct answer is 'D. every computer is capable of playing the role of client, server or both at the same time' because in a peer-to-peer network, each computer can act as both a client and a server. This means that every computer in the network has the ability to request and receive data (client) as well as share and provide data (server) with other computers in the network. This flexibility allows for a decentralized network where each computer can communicate directly with other computers without the need for a central server.