Which HTML 4.01 flavor is necessary to create panes by placing Web pages inside each other?

To create panes by placing web pages inside each other, the appropriate choice is HTML 4.01 Frameset. This flavor of HTML is specifically designed to define a layout using frames, allowing multiple HTML documents to be displayed within a single browser window. Each frame can load a different web page, enabling a multi-pane layout that enhances user navigation without requiring full page reloads.

In contrast, the other HTML 4.01 flavors do not support frames in the same way. HTML 4.01 Transitional allows for certain deprecated elements and attributes from previous versions, mainly for backward compatibility, but does not provide the frame functionality. HTML 4.01 Strict is focused on adhering to web standards and does not include any support for frames at all, as it promotes a cleaner, more accessible, and semantic HTML structure. HTML 4.01 Embedded does not exist as a designated flavor; the common terms used in this context are typically Transitional, Strict, and Frameset.

Thus, for pane creation through embedded web pages, HTML 4.01 Frameset is the necessary choice as it directly facilitates the creation of frames for that purpose.