The file extension of an MS Word document is:

Understanding common file extensions is fundamental for computer literacy. The file extension helps the operating system identify the file type and which program should open it.

• The correct file extension for a modern Microsoft Word document (since Word 2007) is .docx. This format is based on XML and is part of the Office Open XML standard.
• .ppt (A) is the extension for Microsoft PowerPoint presentations.
• .pdf (B) stands for Portable Document Format, a universal file format for presenting documents independently of application software, hardware, and operating systems.
• .txt (D) is used for plain text files, which contain unformatted text.