Which is the correct syntax to change the contents of the HTML element with id 'quiz' again?

The syntax provided in the correct answer effectively uses the DOM (Document Object Model) to access and manipulate an HTML element. The statement document.getElementById('quiz').innerHTML = "New content!"; accurately identifies the HTML element with the specified id, which is 'quiz', and sets its innerHTML property to a new string value, "New content!". This changes the content that is displayed within that specific element on the webpage.

This method is commonly used in web development for dynamically updating content, as getElementById is a widely supported and straightforward way to target a single element by its unique identifier, making it appropriate for this situation. The property innerHTML is used to get or set the HTML markup contained within the element, allowing for the content to be adjusted easily.

The other options contain errors in their syntax or methodology. For instance, using getElementsByTagName would return a collection of elements, not a single element, hence attempting to directly access innerHTML fails unless specifying an index to access the collection. Similarly, using innerHTML as a function call with parentheses, as seen in one of the incorrect options, is not the correct usage for setting content