Which of the following is NOT a popular front-end framework?

The correct choice is based on the understanding of various frameworks used in web development. Bootstrap, Foundation, and Bulma are all widely recognized front-end frameworks used for designing responsive and visually appealing user interfaces. They provide pre-built components, styles, and JavaScript functionality that streamline the development process for the client-side of a web application.

On the other hand, MongoDB is a NoSQL database that is used for backend data storage and management. It does not fall under the category of front-end frameworks, as its primary purpose is to handle data rather than to facilitate user interface design. This distinction clarifies why MongoDB is not considered a popular front-end framework, making it the correct answer to the question regarding which option does not belong to that category.