What is the default file extension of a workbook created in MS Excel 2016?

The correct answer is B, .xlsx. From Excel 2007 onwards the suite moved to the Office Open XML formats, and a plain workbook saves with the .xlsx extension. The x at the end stands for XML, the format in which the sheet contents are stored inside a compressed package, which is why these files are smaller and less prone to corruption than the older ones.

Option A, .xls, was the default extension of Excel 97 through Excel 2003; a 2016 workbook takes it only if the user deliberately saves in the Excel 97-2003 format. Option C, .xlsm, is the macro enabled workbook, used when the file has to carry VBA code, because a plain .xlsx refuses to store macros. Option D, .xltx, is an Excel template, a pattern file from which new workbooks are created rather than a workbook itself; the macro enabled template is .xltm. The exam expects .xlsx whenever the question names Excel 2007 or any later version.