The correct answer is Deployment groups.

Deployment groups are the Azure DevOps feature that lets you register a set of target virtual machines and install a small deployment agent on each machine so Azure Pipelines can push releases directly to those servers. They provide a way to organize machines into a named group, apply tags to select subsets, and run rolling or parallel deployments to the registered hosts.

Environments represents logical deployment environments and can contain resources for multi stage YAML pipelines, but it is not the feature that you use to register and manage a group of target servers in the classic release sense. The question specifically asks about registering and managing target servers as a group which points to deployment groups.

Agent pools are collections of agents that run pipeline jobs and provide compute for builds and deployments, but they do not serve to register or group target servers for release deployments. Agent pools manage the agents that execute tasks rather than the deployment targets themselves.

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