What is the JavaScript syntax to change an element of an HTML page?

The correct syntax for changing an element's content in an HTML page using JavaScript is represented by the choice that utilizes the method getElementById. This method is designed to select a specific HTML element by its unique identifier (ID).

In this case, document.getElementById("demo") targets the element with the ID of "demo". Once this element is selected, accessing its innerHTML property allows for the modification of the content that is displayed within that element. By assigning the string "Hello World!" to the innerHTML, it effectively updates the content of the targeted HTML element to show "Hello World!" on the webpage.

This approach is a fundamental aspect of DOM manipulation in JavaScript, used frequently to update content dynamically based on user interactions or application logic. The other options fail to correctly implement this concept, either by misusing methods or syntax.