Which of these is not a valid method of selecting elements in JavaScript?

The method document.getElementsById(idName) is indeed not a valid method in JavaScript for selecting elements. The correct method to select a single element by its ID is document.getElementById(idName), which retrieves the first element that matches the given ID. The method's name is singular - "Element" rather than "Elements" - reflecting that it returns a single matching element or null if no match is found.

In contrast, the methods document.getElementsByClassName(className), document.getElementsByTagName(tagName), and document.querySelector(selector) are all valid for selecting elements in the DOM. The first two methods return collections of elements (not just one) based on the specified class name or tag name, while document.querySelector(selector) allows for selection based on a CSS selector, returning the first matching element. Each of these methods serves specific selection purposes within the DOM.