The description of wbk format expansion
A wbk object is a backup file created by Microsoft Word, a popular word processor. The file contains a backup copy of a Word document (doc). The wbk resource is used to recover a document file in the event that is deleted or corrupted.
WBK backup files are automatically created when the “Always create backup copy” option is enabled in Word. In Word 2003 you can enable this functionality by choosing Tools -> Options -> Save option. In Word 2007, 2010, 2013, 2016 click Word Options -> Advanced from the Office button on the feed.
If a WBK file is not found on your system, there may also be an auto-save file (ASD) that can recover the Word document.