Which of the following is primarily an XML-based vector image format?

The correct choice is SVG, which stands for Scalable Vector Graphics. This format is specifically designed to describe two-dimensional vector graphics using XML (Extensible Markup Language). SVG allows for high scalability of images without loss of quality, making it ideal for web applications and responsive design where images can be resized depending on the display size.

In terms of its structure, SVG images are defined in an XML format, which means they can be directly manipulated via CSS and JavaScript, making them highly interactive and stylable. This interactivity and scalability set SVG apart from the other options, which are raster formats. In contrast, formats like JPEG, GIF, and PNG are bitmap image formats. They are made up of pixels and can lose clarity when scaled up, as they do not retain the quality that vector images like SVG do.