Which of the following best describes a browser extension?

A browser extension is best described as a tool for enhancing browser functionality. Extensions are small software programs that integrate with web browsers to add features, customize the browsing experience, or improve the performance of the browser. They can provide functionalities such as ad blocking, password management, productivity tools, or even user interface modifications. By enhancing the basic capabilities of a web browser, extensions allow users to tailor their browsing experience to better meet their needs.

Standalone software refers to applications that operate independently, which is not the case with extensions, as they rely on the host browser to function. A feature to surf anonymously could refer to private browsing modes or VPN services, which does not capture the full range of functions that a browser extension provides. Lastly, a programming language is a set of rules and syntax used for developing software, which does not relate to the definition of a browser extension. Therefore, the choice that accurately encapsulates the essence of a browser extension is that it is a tool for enhancing browser functionality.