The extension of MS Excel is:

The traditional file extension for Microsoft Excel workbooks is .xls. This extension was primarily used for Excel files created in versions up to Excel 2003. While newer versions (Excel 2007 and later) introduced the .xlsx extension, .xls remains a widely recognized and compatible format for Excel files.

• .docx is the file extension for Microsoft Word documents.
• .dbx is an extension associated with Outlook Express mail folders or certain database files.
• Therefore, None of these is incorrect, as .xls is a valid and common extension for MS Excel.