Which statement best describes AJAX in web development?

AJAX, which stands for Asynchronous JavaScript and XML, fundamentally enables asynchronous data transfer between the client and server. This means that web applications can send and receive data without having to refresh the entire page, allowing for a more dynamic and interactive user experience. By leveraging JavaScript, AJAX allows the browser to communicate with the server in the background, updating parts of a web page in response to user interactions or other events. This is essential in modern web development, as it enhances the responsiveness of applications and improves overall user satisfaction.

The other statements do not accurately capture the essence of AJAX. Server-side script execution is not a function of AJAX; rather, AJAX interacts with server-side scripts to fetch or send data. Furthermore, AJAX is not a markup language—it relies on JavaScript and sometimes XML or JSON for data exchange. Lastly, while AJAX can work with REST APIs, it is not limited to them; it can be used with other types of web services as well. Therefore, the best description of AJAX is its capability for asynchronous data transfer, which enables more efficient web application interactions.