The correct option is Docker Engine.

Kubernetes historically used Docker Engine as the default container runtime because the kubelet included a dockershim that spoke the Docker Engine API to create and manage containers. That made it the de facto runtime on the majority of clusters for many releases.

Over time the project moved toward the Container Runtime Interface and the dockershim was deprecated and eventually removed in newer Kubernetes releases. This change means that while Docker Engine is the traditional answer, newer exams often focus on CRI compatible runtimes instead.

CRI-O is a CRI implementation designed to work with Kubernetes and it is a supported and popular alternative. It was not the traditional default however so it is not the correct choice for a question about the historical default.

LXC is a Linux container technology but it is not a standard Kubernetes container runtime and Kubernetes does not use LXC as its default runtime. That makes LXC incorrect for this question.

containerd runtime is a modern, CRI compatible runtime and it is widely used today and often recommended for new clusters. It was not the traditional default historically so containerd runtime is not the correct answer to a question asking which runtime was the traditional default.

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