This is because according to VMware documentation, this is what an administrator must register as an external component to complete this task of configuring scheduled backups for the SDDC Manager. SFTP server is one of the supported backup targets for SDDC Manager backups. The administrator can register an SFTP server by navigating to the SDDC Manager UI > Administration > Backup > Site Settings and clicking Register External.

D.
1 Decommission the failed host
2. Remove the host using cluster APIs.
3. Commission the new host with the correct network.
4. Add the newly commissioned host to the cluster using cluster APIs.
Explanation:
This is because according to VMware documentation1, these are the steps to replace a failed host in a stretched cluster:
Run the compact cluster API to remove any stale data from vSAN.
Decommission the host to be removed using SDDC Manager UI or API.
Commission the replacement host to the same network pool as the removed host using SDDC Manager UI or API.
Add the newly commissioned host to the cluster using SDDC Manager UI or API.
According to the VMware documentation, when replacing a failed host in a stretched cluster, the first step is to decommission the failed host. This should be followed by removing the host using cluster APIs, commissioning the new host with the correct network, and then adding the newly commissioned host to the cluster using cluster APIs.

According to VMware Cloud Foundation Architecture Poster, VCF with NSX-T Federation provides network high availability across workloads in two data center locations by synchronizing network configuration and state across sites. It also simplifies day 2 operations by providing centralized management and policy enforcement across sites. It also reduces RTO by enabling fast failover of network services between sites.

https://docs.vmware.com/en/VMware-Cloud-Foundation/4.5/vcf-admin/GUID-CDEEF4C6-7DFC-4EB5-B5F5-3C41230926F9.html

Explanation
This is because when deploying a stretched cluster in a VMware Cloud Foundation environment, you must ensure that:
* The vSAN network, the host overlay network and the vMotion network are stretched across both availability zones12. These networks enable data replication, communication and migration between hosts in different sites.
* The host overlay network has DHCP enabled to provide IP addresses to hosts1. This simplifies the configuration and management of hosts in different sites.
* Each vSAN stretched cluster has one witness host deployed in a separate location from both availability zones12. The witness host acts as a tie-breaker in case of a site failure or split-brain scenario.
https://docs.vmware.com/en/VMware-Cloud-Foundation/4.5/vcf-admin/GUID-7B4CC729-20BD-4CC9-B855-B