Which of the following is NOT a valid Microsoft Excel file format?

The correct answer is D: .pdf. While Microsoft Excel can export data to a PDF (Portable Document Format) file, .pdf is not a native or valid Microsoft Excel *file format* for storing editable spreadsheet data. PDF is a universal file format used to present documents in a manner independent of application software, hardware, and operating systems. When you save an Excel sheet as a PDF, it essentially creates a snapshot or print-ready version, not a functional spreadsheet file that can be fully reopened and edited in Excel with all its formulas and sheet structure intact.

• A: .xlsx is incorrect. .xlsx is the primary and default file format for Microsoft Excel documents starting from Excel 2007. It is an XML-based format that supports all modern Excel features.
• B: .csv is incorrect. .csv (Comma Separated Values) is a widely used plain-text file format that stores tabular data in rows and columns, where each value is separated by a comma. Excel can open, edit, and save .csv files, making it a valid and common format for data exchange with Excel.
• C: .xls is incorrect. .xls is the traditional file format for Microsoft Excel workbooks from Excel 97 to Excel 2003. Although superseded by .xlsx, it is still a valid and recognizable Excel file format that modern Excel versions can open and save.