The correct answer isB: CustomResourceDefinition (CRD). Kubernetes is designed to be extensible. ACRDlets you define your own resource types (custom API objects) that behave like native Kubernetes resources: they can be created with YAML, stored in etcd, retrieved via the API server, and managed using kubectl. For example, operators commonly define CRDs such as Databases, RedisClusters, or Certificates to model higher-level application concepts.
A CRD extends the API by adding a new kind under a group/version (e.g., example.com/v1). You typically pair CRDs with acontroller(often called an operator) that watches these custom objects and reconciles real-world resources (Deployments, StatefulSets, cloud resources) to match the desired state specified in the CRD instances. This is the same control-loop pattern used for built-in controllers—just applied to your custom domain.
Why the other options aren’t correct: ConfigMaps store configuration data but do not add new API types. A MutatingAdmissionWebhook can modify or validate requests for existing resources, but it doesn’t define new API kinds; it enforces policy or injects defaults. Kustomize is a manifest customization tool (patch/overlay) and doesn’t extend the Kubernetes API surface.
CRDs are foundational to much of the Kubernetes ecosystem: cert-manager, Argo, Istio, and many operators rely heavily on CRDs. They also support schema validation via OpenAPI v3 schemas, which improves safety and tooling (better error messages, IDE hints). Therefore, the mechanism for extending the Kubernetes API isCustomResourceDefinition, optionB.
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