Explanation:

HTML stands for Hypertext Markup Language, and it is the standard markup language used for creating webpages. The source code of an HTML document contains the structure and content of the webpage, including elements such as text, images, links, and formatting.

To view the source code of an HTML document, you can use a text editor that allows you to open and edit plain text files. Here are some commonly used text editors:

1. Notepad:
Notepad is a basic text editor that is available on most Windows operating systems. It allows you to open and edit plain text files, including HTML files. To view the source code of an HTML document in Notepad, you can simply open the HTML file with Notepad and the code will be displayed.

2. Powerpoint:
Powerpoint is a presentation software that is primarily used for creating slideshows. It is not designed for viewing or editing HTML source code. Therefore, you cannot view the source code of an HTML document in Powerpoint.

3. Excel:
Excel is a spreadsheet software that is used for organizing and analyzing data. Similar to Powerpoint, Excel is not suitable for viewing or editing HTML source code. Therefore, you cannot view the source code of an HTML document in Excel.

4. Paint:
Paint is a basic image editing software that allows you to create and edit images. It does not support the viewing or editing of HTML source code. Therefore, you cannot view the source code of an HTML document in Paint.

In conclusion, the correct answer is option 'A' - Notepad. Notepad is a text editor that allows you to view and edit plain text files, including HTML files. It is a commonly used tool for viewing the source code of HTML documents.