The correct option is Docker Engine.

Docker Engine was the container runtime that Kubernetes originally integrated with and used to schedule and run containers. The kubelet talked to Docker through a shim called dockershim which translated Kubernetes container operations into Docker API calls so Docker Engine was the first runtime relied upon by the project.

Dockershim was later deprecated and removed from the Kubernetes code base and Kubernetes moved to the Container Runtime Interface which lets runtimes such as containerd and CRI-O be used directly. This historical note explains why Docker Engine was first but why newer exam items often emphasize CRI and CRI-compatible runtimes.

containerd is a CRI-compatible runtime that grew out of the Docker ecosystem and it became widely used later. It was not the runtime Kubernetes first relied on because the original integration was with Docker Engine through dockershim.

gVisor is a sandboxing runtime from Google that focuses on additional isolation. It is an optional secure runtime and it was never the default or the original runtime for Kubernetes.

CRI-O implements the Container Runtime Interface to run OCI containers for Kubernetes without Docker. It was developed after the CRI design and so it was not the first runtime that Kubernetes relied on.

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