The correct choice is (c) <slot>
Easiest explanation: <shadow> is obsolete part of web component used to shadow DOM. The obsolete part of web components is <content>. <slot> element in HTML is a part of Web Components technology suite. It is a placeholder inside web component that you can fill with own markup that lets you create separate DOM trees and present them together. <element> is one of part of web components, used to custom DOM elements.
