Which set of element tags is used to create the highest level heading?

The use of the <h1>...</h1> tags is specifically designed to denote the highest level heading in HTML. These tags provide the most significant emphasis and indicate the primary subject of the webpage. By utilizing the <h1> tag, web developers are not only establishing a hierarchical structure for the content but also enhancing the accessibility and search engine optimization (SEO) of the webpage. Search engines prioritize the content within <h1> tags as it helps them understand the main topic of the page, which can be crucial for indexing and ranking.

Other heading tags, such as <h6>...</h6>, represent lower levels of headings, with <h6> being the least significant. The <header>...</header> tag serves a different purpose by defining a container for introductory content or navigational links, rather than functioning as a heading level itself. There is no <h9> tag in HTML, making that option invalid. Thus, the <h1> tag is essential for establishing the primary heading, cementing its role as the correct choice in this context.