Conquer the CKAD Challenge 2025: Unleash Your Kubernetes Power!

The command to use for applying a configuration file to a Kubernetes resource is "kubectl apply -f <config-file>.yaml." This command is specifically designed to update resources in the Kubernetes cluster or create them if they do not already exist. The `apply` operation ensures that the desired state specified in the configuration file is configured in the cluster. It intelligently handles updates by merging the changes, which is particularly useful for managing resources in environments where configurations are continually evolving.

The use of "-f" indicates that a file should be specified, pointing to the configuration file in YAML format that contains the desired state of the Kubernetes resource.

In contrast, the other command options do not correctly represent the functionality needed to apply a configuration file. "kubectl deploy" and "kubectl update" do not exist as valid commands for applying resources. "kubectl configure" is also not valid in this context, as it pertains to configuring the Kubernetes command-line tool and its context rather than applying resource configurations.