What is the default extension for an Excel workbook file?

The default extension for an Excel workbook file is .xlsx. This format was introduced with Excel 2007 and represents the standard file type for workbooks created in Excel since then. The .xlsx format is based on the Office Open XML standard, which allows for improved data recovery and more efficient file sizes compared to older formats.

In contrast, .xls is the extension used for workbooks created in Excel 97-2003, while .xlsm indicates a macro-enabled workbook that allows users to run macros within the file. .xlsb is a binary workbook format that is typically used for larger files because it can reduce file size and increase performance, but it is not the default format. Therefore, .xlsx is the correct and most commonly used file extension for modern Excel workbooks.