Final answer:
A Virtual Private Network (VPN) is a secure network built on top of a public infrastructure, designed to provide a private and controlled network environment for its users.
Step-by-step explanation:
A Virtual Private Network (VPN) is a network with security and controlled access for a private group but built on top of a public network. The correct answer to the question is D. Virtual private network. A VPN allows users to send and receive data across shared or public networks as if their computing devices were directly connected to the private network, providing functionalities, security, and management policies of the private network. This is done by establishing a virtual point-to-point connection through the use of dedicated connections, virtual tunneling protocols, or traffic encryption.