Block-level elements begin on a new line.

Block-level elements indeed begin on a new line when rendered in a web page, which is a fundamental aspect of how HTML structures content. This means that when a block-level element like a <div>, <h1>, or <p> is used, it will take up the full width available in its container, effectively creating a distinct section that causes the following content to start below it on a new line.

This behavior is a crucial concept in web development, as it helps create a logical and orderly layout for web pages. Understanding the nature of block-level elements is essential for effectively utilizing CSS and managing the flow of content within a webpage.

The other choices present scenarios that do not align with the standard behavior of block-level elements in modern web development. Therefore, affirming that block-level elements start on a new line is accurate and reflects how they contribute to the overall layout of a webpage.