Explanation:
Box 1: No
On January 15, 2021, App1 will have only one backup in storage.
On January 15th you will have 9 backups as 0 day retention is defined as indefinite.
Retention: Set to 0 for indefinite retention.
Box 2: No
On February 6, 2021, you can access the backup of the App2 test slot from January 15, 2021.
Backup in the questions only includes the production slot. So no backup policy for test slots.
Box 3: Yes
On January 15, 2021, you can restore the App2 production slot backup from January 6 to the App2 test slot.
You can restore the production backup to any slot or new deployment slot.
References:
https://learn.microsoft.com/en-us/azure/app-service/manage-backup?tabs=portal