Explanation
To expand disk group #2 to have three capacity devices, the vSAN administrator should add the new capacity device to the disk group and vSAN will automatically rebalance. This action allows the administrator to increase the storage capacity of the disk group without disrupting any ongoing operations or evacuating any data. vSAN will automatically distribute data across all devices in the disk group to balance performance and utilization. The other options are not correct. Creating a new disk group with a single capacity device and then migrating the existing capacity devices is not necessary, as it would require more steps and resources than adding a device to an existing disk group. Putting the entire ESXi host or the disk group in maintenance mode and evacuating all data is not required, as it would cause downtime and data movement that could be avoided by adding a device to an existing disk group. References: Add Devices to the Disk Group; Expanding a vSAN Cluster