Which format is used to set up the structure of a webpage?

The correct choice is HTML, which stands for Hypertext Markup Language. HTML is the foundational language used to create and structure content on the web. It defines the elements on a webpage such as headers, paragraphs, links, images, lists, and other content. Through the use of tags, HTML establishes the hierarchy and organization of the webpage, delineating what is a title, what is a section, and how different elements relate to each other.

CSS (Cascading Style Sheets) comes into play after the structure has been created with HTML. It is used for styling the visual presentation of the webpage—defining colors, fonts, layouts, and responsive designs. JavaScript adds interactivity and dynamic behavior to a webpage, such as responding to user actions or manipulating the displayed content, but it does not establish the basic structure.

Thus, when it comes to setting up the backbone and layout of a webpage, HTML is the essential format that serves this purpose.