Is there are provision for quarantining DLL file?
If so how is it accessed to mark such a file as safe?
I am going to guess and say the whole file :-)
You can remove files selectively if you wish to - on the right hand side, after selecting a quarantined threat, there will be an "options" button (small, and in blue), click on that and you should have three options, one of which is to remove the file from quarantine. this will thus allow you to remove selectively from quarantine (and hence per my guess) from your pc.
Matt
First all
click on the norton icon on the taskbar , and go to the top and click on the quarantine section , and there you can see the history if you see something click on clean entries , but sometimes is just entries nothing on risk , I recommended to do a full scan with malawarebytes or norton power eraser , to make sure you're clean.
Amaneses54 wants to know if "clearing" the quarantined item or items RESTORES the item or REMOVES THE ITEM FROM THE COMPUTER. I want to know the same thing!
Clearing history removes both the history and removes (deletes) the files from quarantine. It does NOT restore the file as that can only be done by explicitly choosing to do so.
If you have files in quarantine you can see this in action by finding the QBackup folder on your C drive and then selecting the Quarantine list and clicking on Clear Entries and then OK. Please note: be sure you really want to do this as this will permanently remove the files and you can no longer restore from quarantine.
You will see the directories underneath QBackup disappearing before your eyes. 
Note: you will have to modify the Windows Explorer view properties and check the box for Show hidden files and folders and uncheck the box which says Hide protected operating system files.
Hope this helps.
Allen