Microsoft Office applications each save their files with a characteristic extension, and since Office 2007 these have used the Open XML format, signalled by the letter x at the end of the extension. This XML-based format stores the document as a compressed package of XML parts, which makes files smaller, more robust against corruption and easier for other programs to read.

For Microsoft PowerPoint 2021, the default extension for a presentation is .pptx, so this is the correct answer. (The letters ppt stand for PowerPoint presentation, and the trailing x marks the modern Open XML format that replaced the older binary .ppt used up to Office 2003.)

The other extensions belong to different applications and file types:

• .xlsx — an Excel workbook (spreadsheet), not a slideshow.
• .docx — a Word document (word processing), used for letters and reports.
• .pdf — Portable Document Format, a fixed-layout format from Adobe used for sharing/printing; PowerPoint can export to PDF, but that is not the native format in which a presentation is saved.

Because none of these three is the working format PowerPoint uses to store an editable presentation, .pptx is the only correct choice.