Which of the following tools could be classified as a plugin?

A browser extension that modifies web pages is classified as a plugin because it adds specific features or functionalities to a web browser, enhancing the user experience. Plugins are designed to integrate seamlessly with existing software, allowing the user to personalize or augment the capabilities of the main application—in this case, the web browser.

Browser extensions can perform a variety of tasks, such as blocking ads, improving privacy, or altering the appearance of web pages. They operate within the context of the browser and can interact with the content being displayed. This interoperability is a hallmark of what defines a plugin.

Other options listed, such as a virus scanner for an operating system, photo editing software, and a standalone application for video conferencing, do not fit the definition of a plugin. These options are either independent applications or security tools that do not enhance or augment another existing software environment in the way that a browser extension does.