Which version of HTML extends features to process video and audio without proprietary plug-ins?

HTML5 is the correct answer as it introduced native support for audio and video content in web pages, allowing for multimedia to be played directly in the browser without the need for proprietary plug-ins like Flash. This was a significant advancement over previous versions of HTML, which required third-party software for multimedia elements.

In HTML5, specific elements such as <audio> and <video> were introduced, providing a straightforward way for developers to embed media files. This change not only simplified the coding process for web developers but also enhanced the user experience by enabling seamless playback of video and audio on a wide variety of devices and browsers.

In contrast, earlier versions like HTML4 and XHTML did not have built-in support for multimedia and relied on external players, which could lead to compatibility issues and a fragmented user experience. HTML3.2 also lacked these multimedia capabilities, further emphasizing the innovation that HTML5 brought to web development.