Conducting a full backup of the server before initiating the upgrade process A vital step in a multi-node server upgrade for Tableau Server is conducting a full backup of the server before starting the upgrade. This ensures that data is secured and can be restored in case of any is-sues during the upgrade, maintaining data integrity and allowing for recovery if needed. Option A is incorrect because disabling user access entirely may not be necessary if the upgrade is staged properly. Option C is incorrect as immediate updates to the operating systems of the server nodes may not be required and should be carefully planned. Option D is incorrect because removing nodes from the cluster can impact the performance and redundancy of the server, and is not a recommended practice for an upgrade.