To ensure an event listener fires only once, two primary methods can be used. The first is to pass an options object as the third argument to addEventListener with the once property set to true. This instructs the browser to automatically remove the listener after it has been invoked. The second method is to explicitly call removeEventListener from within the event handler function itself, which achieves the same result by manually cleaning up the listener after its first execution.

A. event.stopPropagation() prevents the event from bubbling up the DOM tree to parent elements, but it does not remove the listener itself.

B. The Event object does not have a first property. This is not a standard way to track event listener invocations.

1. MDN Web Docs, "EventTarget.addEventListener()", Section: "The options object". The documentation details the once option: "A boolean value indicating that the listener should be invoked at most once after being added. If true, the listener would be automatically removed when invoked."

2. MDN Web Docs, "EventTarget.removeEventListener()", Syntax section. This page explains how to remove an event listener that was previously added with addEventListener(). Using it inside the handler is a common pattern for one-time events.