The correct answer is (d) <article>
Easy explanation: A self-contained composition in document, application, page or site that is intended to be independently distributable is represented by <article> element. Some of the examples are a magazine or newspaper article or a forum post, or a blog entry. <nav> element contains all the nevigation links. Footer typically contains information about author of the section, links to related documents or copyright data. An introductory content lies in <header>.