Notepad is a simple text editor that comes with Windows operating system. It is used for creating and editing plain text files. The extension of a file determines the type of file it is and which program is used to open it. The extension of Notepad files is ".txt".

Explanation:

Extension of a file: The extension of a file is a set of characters that follow the last dot in the file name. It determines the type of file and which program is used to open it. For example, a file with the extension ".doc" is a Microsoft Word document, and it can be opened using Microsoft Word.

Notepad: Notepad is a simple text editor that is included with the Windows operating system. It is used for creating and editing plain text files, such as HTML, CSS, JavaScript, and other programming languages.

Extension of Notepad files: The extension of Notepad files is ".txt". This extension indicates that the file is a plain text file and can be opened using any text editor, such as Notepad, Wordpad, or any other third-party text editor.

Other options:

b) .xls: The extension ".xls" is used for Microsoft Excel files. These files contain data or information in a spreadsheet format.

c) .ppt: The extension ".ppt" is used for Microsoft PowerPoint files. These files contain presentation slides with text, images, and multimedia elements.

d) .bmp: The extension ".bmp" is used for bitmap image files. These files contain an image in a bitmap format.