Final answer:
The mini-applications referenced by the \u003cobject\u003e tags in HTML5 could have been created using Adobe Flash or Java, but modern practices use HTML5 Canvas, SVG, or WebGL technologies.
Step-by-step explanation:
If you're reviewing HTML5 markup code and you come across \u003cobject\u003e tags that reference external stand-alone, mini-applications, the language that your co-worker most likely used to create these mini-applications is Adobe Flash (ActionScript) or Java (Java Applets). However, since Adobe Flash support has officially ended, and modern browsers do not support Java Applets anymore, it is possible that the objects are using newer technologies such as HTML5 Canvas, SVG, or WebGL which do not require external plugins to run and are native to the HTML5 specification.