A. Create an Outlook rule that forwards all the items in each public folder to a compliance mailbox.
B. Place an In-Place Hold on all the mailboxes of the public folders.
C. Increase the frequency of the backups.
D. Modify the public folder permissions.
Explanation:
References: https://docs.microsoft.com/en-us/exchange/security-and-compliance/in-place-and-litigation-holds#placingpublic-folders-on-hold