Which framework assists designers with adaptive page layout?

React is a JavaScript library primarily used for building user interfaces, particularly for single-page applications. While it facilitates component-based architecture, it does not inherently provide a layout system or responsive design features like Bootstrap. React focuses on rendering UI components rather than managing page layouts.

Knockout is a JavaScript library that implements the Model-View-ViewModel (MVVM) pattern. It is useful for creating dynamic UIs by managing data binding and dependencies, but it lacks built-in responsive design capabilities or a grid system, making it less suited for adaptive page layouts compared to Bootstrap.

Modernizr is a JavaScript library that detects HTML5 and CSS3 features in a user's browser. While it helps developers implement fallbacks for unsupported features, it does not provide layout tools or responsive design frameworks. Its role is more about feature detection than layout management.