According to the vRealize Operations User Guide1, the action that the administrator should take to accomplish the goal of making sure that the existing vRealize Operations Manager instance can monitor a newly deployed vSAN cluster within an existing vCenter Server is to enable vSAN monitoring under the existing vCenter Server Cloud Account. A vCenter Server Cloud Account is a configuration that defines the connection details and credentials of the vCenter Server instance that manages the vSphere components, such as clusters, hosts, datastores, and virtual machines. By enabling vSAN monitoring under the vCenter Server Cloud Account, the administrator can allow vRealize Operations Manager to collect data from the vSAN cluster and monitor the health, performance, capacity, and configuration of the vSAN components, such as disk groups, disks, fault domains, and storage policies. The guide provides the steps to enable vSAN monitoring under the vCenter Server Cloud Account using the vRealize Operations Manager user interface1. The other options are not correct because:

Adding vSAN cluster under Other Accounts is not a valid action, as there is no such option in the vRealize Operations Manager user interface. The Other Accounts option under Administration is used to add accounts for other types of cloud services, such as AWS, Azure, Google Cloud, or Kubernetes, but not for vSAN cluster1.

Downloading and installing vRealize Operations Management Pack for vSAN is not a required action, as vRealize Operations Manager already supports vSAN monitoring natively without the need of any additional management pack. The vRealize Operations Management Pack for vSAN is an optional extension that provides additional dashboards, alerts, and reports for vSAN monitoring, but it is not necessary to accomplish the goal2.

Adding vSAN cluster under Cloud Accounts is not a valid action, as there is no such option in the vRealize Operations Manager user interface. The Cloud Accounts option under Administration is used to add accounts for VMware Cloud services, such as VMware Cloud on AWS, VMware Cloud Director, or NSX-T, but not for vSAN cluster1. References: 1: vRealize Operations User Guide, pages 17-18, 21-222: vRealize Operations Management Pack for vSAN, Overview section.