Explanation

Step 1: Perform a differential database backup...
Step 2: Perform a tail-log backup...
A tail-log backup captures any log records that have not yet been backed up (the tail of the log) to prevent work loss and to keep the log chain intact. Before you can recover a SQL Server database to its latest point in time, you must back up the tail of its transaction log. The tail-log backup will be the last backup of interest in the recovery plan for the database.
Step 3:
The PRIMARY and FG2 for DB1 are damaged. FG1 and FG3 are intact.
Step 4:
Transaction logs are backed up to a backup set named TLogBackup.
Step 5:
The PRIMARY and FG2 for DB1 are damaged.
References:
https://docs.microsoft.com/en-us/sql/relational-databases/backup-restore/restore-files-and-filegroups-sql-server?v
https://docs.microsoft.com/en-us/sql/relational-databases/backup-restore/tail-log-backups-sql-server?view=sql-se