Which file extension is usually used when saving an MS Word document?

.docx → Default extension for Word documents in MS Word 2007 and later (including MS Word 365).
.doc → Older extension used in Word versions prior to 2007.
Other related formats:
.pdf → Portable Document Format (when exporting/saving as PDF).
.rtf → Rich Text Format.
.txt → Plain text files.
Key Takeaway
The standard and most common extension