The type of framework that is popular for building responsive web applications is Bootstrap. Bootstrap is an open-source front-end framework that provides HTML, CSS, and JavaScript components for designing responsive, mobile-first websites. It includes a grid system, pre-defined styles, and customizable UI components, allowing developers to create visually appealing and functional web applications that adapt to various screen sizes. Bootstrap enhances the user experience by promoting consistent design and improved accessibility, making it a go-to choice for modern web development.