Which of the following events is fired when the user opens or closes the DETAILS element?

The correct answer is C) toggle.

<details id="details">
 <summary>toggle event</summary>
</details>
<script>
 let details = document.querySelector("#details")
 
 details.addEventListener("toggle", function() {
    details.firstChild.textContent = "done"
 })
</script>

The toggle event is fired when the user opens or closes the <details> element. This event can be used to trigger a variety of actions, such as displaying additional information or showing a different content.