Which of the following tools serves as an example of an agent-based configuration management tool?

Chef is an example of an agent-based configuration management tool because it relies on agents that are installed on the managed systems. These agents interact with a centralized server to fetch configurations and apply them to ensure that the systems are in the desired state. This approach allows for real-time monitoring and enforcement of the configuration settings as specified in the Chef recipes.

In contrast, Ansible operates as an agentless automation tool, using SSH to communicate with the target systems, thereby not requiring agents to be installed on those systems. Terraform is a tool primarily focused on infrastructure provisioning and does not manage the configuration of servers in the same way that Chef does. Docker, while a powerful tool for containerization, does not function as a configuration management tool and is more about packaging applications within containers. Hence, among the options, Chef stands out as the agent-based configuration tool.