The correct answer is 'C. WK1' because Lotus 1-2-3 is a spreadsheet program developed by Lotus Software, and files created with this program typically have the file extension '.WK1'. This extension is specific to Lotus 1-2-3 files and is commonly used to identify and open these files in the appropriate software. Therefore, 'WK1' is the correct extension for files created with Lotus 1-2-3.