According to the web search results, the most efficient action an administrator should take to ensure a supported state when adding a node with a higher AOS release to an existing cluster is to perform a standalone reimage of the new node and then add it to the cluster1. This way, the new node will have the same AOS version as the existing nodes, and there will be no compatibility issues or downtime. Therefore, option C is correct, while options A, B, and D are not.