Which of the following is a popular JavaScript framework?

React is a popular JavaScript framework designed for building user interfaces, particularly for single-page applications. It allows developers to create large web applications that can change data without needing to reload the page. This creates a smooth user experience that is essential in modern web development.

React's component-based architecture also promotes reusability and improves maintainability of code, allowing developers to break down complex interfaces into smaller, manageable pieces. Additionally, React uses a virtual DOM to enhance performance by minimizing direct interactions with the actual DOM, which can be time-consuming.

While other options like Bootstrap and jQuery are important tools in web development, they serve different purposes. Bootstrap is a front-end framework for designing responsive web pages, and jQuery is a library that simplifies HTML document traversing, event handling, animating, and Ajax interactions. CSS, on the other hand, is a stylesheet language used for describing the presentation of a document written in HTML or XML, not a framework. Thus, React stands out as the correct answer in the context of popular JavaScript frameworks.