An administrator has to perform maintenance on one of the hosts in a three-node vSAN...

 To perform maintenance on one of the hosts in a three-node vSAN cluster with the best availability for the VMs with the least effort and data transfer, the maintenance mode option that should be used is Ensure accessibility. This option migrates only enough components to ensure that all accessible VMs remain accessible, but does not guarantee full data redundancy or policy compliance. This option is also the only evacuation mode available for a three-node cluster or a cluster with three fault domains, as there are not enough hosts to perform full data migration or re-protection after a failure. The other options are not correct. Migrating all VMs and their storage from the host to a different storage system or a different vSphere cluster would require more effort and data transfer than using Ensure accessibility, as well as additional resources and configuration steps. Full data migration is not possible in a three-nodecluster, as it would require at least four hosts to evacuate all data from one host and maintain full redundancy and policy compliance. References: Place a Member of vSAN Cluster in Maintenance Mode; Working with Maintenance Mode