Right answer is (a) <element>
To elaborate: <element> is HTML obsolete element. It was the part of Web Components specification. It was intended to be used to define new custom DOM elements. It was removed in favor of JavaScript-driven approach for creating new custom elements. <shadow> is obsolete part of web component used to shadow DOM. <slot> element in HTML is a part of Web Components technology suite. The obsolete part of web components is <content>.