File extensions indicate the type of file and the program that can typically open it.
The .txt extension stands for text. It denotes a plain text file, which contains only unformatted text characters.
Let's look at the applications and their default file extensions:
(A) MS-WORD: Primarily uses .doc or .docx for formatted documents.
(B) MS-ACCESS: Uses .accdb for database files.
(C) NOTEPAD: This is the default plain text editor in Windows. It creates and saves files with the .txt extension.
(D) MS-EXCEL: Uses .xls or .xlsx for spreadsheet files.
Therefore, a .txt file belongs to Notepad.