Final answer:
Option (C), A Microsoft Word document typically has the extension .doc, which is the set of three letters at the end of its file name.
Step-by-step explanation:
The three letters at the end of a Microsoft Word document name indicate the file extension, which tells you what type of file it is. For a Word document, the correct file extension is .doc. Option A (.xls) is for Microsoft Excel documents, Option B (.ppt) is for Microsoft PowerPoint presentations, Option D (.pdf) stands for Portable Document Format which is not specific to Microsoft Office. Therefore, the answer to the question, 'A Word document has which three letters at the end of its name?' is C) .doc.