I think that the current free resources are probably sufficient — especially if you’ve been working with Kubernetes for a while or for your day-job.
You do not have a choice on distribution, but it should not have an impact on the exam.
As for Kubernetes version, the CKA Tips (https://www.cncf.io/certification/tips) document will let you know the current Kubernetes version (v1.9.1 as of this moment)
Browsing the Kubernetes documentation and other public general resources (for example k8s the hard way or Walid Shaari’s repo) are completely allowed. This includes copy/pasting from those repos.
That said, what the exam facilitators want to prevent is people having knowledge about specific…