What is the correct JavaScript syntax to change an element's HTML?

The syntax that changes an element's HTML correctly utilizes the getElementById method, which is a standard way in JavaScript to select an HTML element by its unique ID.

The method document.getElementById("demo") retrieves the element with the ID of "demo" in the document. Once this element is selected, the property innerHTML can be set to a new value, in this case, the string "Hello World!". The innerHTML property is commonly used to get or set the HTML content inside an element, making it an appropriate choice for modifying the inner content of an element.

Other options demonstrate incorrect approaches or syntax. They either misuse the method for element selection or employ nonexistent properties and methods, which clarifies why the correct syntax of option D stands out as the only accurate choice.