The correct answer is C: metrics.k8s.io. Kubernetes’ metrics-server is the standard component that provides resource metrics (primarily CPU and memory) for nodes and pods. It aggregates this information (sourced from kubelet/cAdvisor) and serves it through the Kubernetes aggregated API under the group metrics.k8s.io. This is what enables commands like kubectl top nodes and kubectl top pods, and it is also a key data source for autoscaling with the Horizontal Pod Autoscaler (HPA) when scaling on CPU/memory utilization.
Why the other options are wrong:
custom.k8s.io is not the standard API group for metrics-server resource metrics. Custom metrics are typically served through the custom metrics API (commonly custom.metrics.k8s.io) via adapters (e.g., Prometheus Adapter), not metrics-server.
resources.k8s.io is not the metrics-server API group.
cadvisor.k8s.io is not exposed as a Kubernetes aggregated metrics API. cAdvisor is a component integrated into kubelet that provides container stats, but metrics-server is the thing that exposes the aggregated Kubernetes metrics API, and the canonical group is metrics.k8s.io.
Operationally, it’s important to understand the boundary: metrics-server provides basic resource metrics suitable for core autoscaling and “top” views, but it is not a full observability system (it does not store long-term metrics history like Prometheus). For richer metrics (SLOs, application metrics, long-term trending), teams typically deploy Prometheus or a managed monitoring backend. Still, when the question asks specifically which API exposes metrics-server data, the answer is definitively metrics.k8s.io.
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