Final answer:
The correct extension for Microsoft Excel 2013 workbooks is .xlsx, which supports data on worksheets, charts, and macros, and is the default file format from Excel 2007 and later versions.
Step-by-step explanation:
The extension for Microsoft Excel 2013 workbooks is .xlsx. This file extension represents an Excel workbook file in the Open XML format, which is used by Microsoft Excel 2007 and later versions. The .xlsx format includes the data on worksheets, charts, and macros that you may have added. Before the introduction of the Open XML formats with Excel 2007, the default file extension was .xls, which was used by earlier versions of Excel up to Excel 2003. The .xls format is based on Binary Interchange File Format (BIFF).
When you save a document in Excel 2013, it will default to the .xlsx extension, unless you choose to save it in a different format, such as the older .xls format or the macro-enabled .xlsm format, which would be necessary if you have macros embedded in your workbook. The .xlsx format helps to facilitate data interoperability between Microsoft software and other platforms, as it is based on the XML structure which is more open and can be read by various software applications.