What technology allows a VM to be isolated within a private virtual network?

The correct choice indicates the technology that enables a Virtual Machine (VM) to be isolated within a private virtual network is Network Address Translation (NAT). NAT functions by allowing multiple virtual machines to communicate over the same public IP address while still keeping them isolated from each other and from external networks. This is particularly useful in virtualized environments where security and network management are concerns.

With NAT, each VM can have its own private IP address that is not visible to the outside world. Instead, when data needs to be sent or received, NAT translates these addresses to the public one, ensuring that while the VMs can communicate externally for services (internet access, for example), they remain secure within their private network. This separation helps ensure that the traffic between the VMs does not directly interface with outside networks, preserving their isolation and enhancing security.

The other options present functionalities that differ from the isolation feature provided by NAT. A firewall primarily focuses on filtering incoming and outgoing traffic, a VPN creates secure encrypted connections over public networks, and bridging connects two or more networks to make them act as a single network. None of these effectively isolates VMs within a private virtual network like NAT does, making it the most suitable technology for the purpose described in the question.