Notepad is a simple text editor that comes pre-installed in Windows operating systems. It is commonly used to create and edit plain text files. The extension of a file represents the format or type of the file. In the case of Notepad, the extension is ".txt".

Explanation:

The correct answer is option 'A', which states that the extension of Notepad is ".txt". Here is the detailed explanation:

1. Notepad:
- Notepad is a basic text editor that allows users to create and edit plain text files.
- It is a lightweight program that is included with the Windows operating system.
- Notepad does not support advanced formatting or features found in word processors or other specialized text editors.

2. File Extensions:
- File extensions are a way to identify the format or type of a file.
- They typically consist of three or four characters following a period (dot) in the file name.
- The extension gives information to the operating system and associated programs about how to handle the file.

3. ".txt" Extension:
- The ".txt" extension is commonly used for plain text files.
- Plain text files contain unformatted text and do not include any special formatting or embedded objects.
- Notepad saves files with the ".txt" extension by default.

4. Other Options:
- Option 'B' states that the extension of Notepad is ".xls". This is incorrect because ".xls" is the extension for Microsoft Excel spreadsheet files, not text files.
- Option 'C' states that the extension of Notepad is ".ppt". This is incorrect because ".ppt" is the extension for Microsoft PowerPoint presentation files, not text files.
- Option 'D' states that the extension of Notepad is ".bmp". This is incorrect because ".bmp" is the extension for bitmap image files, not text files.

In conclusion, the correct extension for Notepad files is ".txt". This extension indicates that the file is a plain text file and can be opened and edited using Notepad or other text editors.