What is the default file extension for an Excel workbook?

The default file extension for an Excel workbook is .xlsx. This extension was introduced with Excel 2007 and is part of the Office Open XML format. It is designed to allow greater data management and storage efficiency, as well as improved accessibility across different platforms. The .xlsx format supports features such as larger file sizes, increased rows and columns, and the ability to include more complex data types and structures than previous versions.

In contrast, .xls was used by earlier versions of Excel prior to 2007, which is less efficient and has limitations on data capacity. The .docx extension is associated with Microsoft Word documents, while .pptx is used for PowerPoint presentations. Hence, these do not pertain to Excel workbooks, confirming that the .xlsx extension is indeed the correct answer.