Which of the following is a quality of solutions created by web browsers?

Not all web browsers are proprietary; many popular browsers are open-source projects, making this an inaccurate generalization.

While web content displayed by browsers is designed to be cross-platform, the web browser application itself is developed and runs on a specific operating system platform.

A web browser, as an application, is specifically compiled and designed to run on a single operating system platform (e.g., Windows, macOS, Linux, iOS, Android), making the browser itself a "single platform" solution. The browser's execution environment is intrinsically tied to the specific operating system it is installed on.

Licensing models, such as site licensing, are commercial aspects and not a fundamental technical quality of the solutions created by web browsers.