Correct choice is (a) <plaintext>
Easy explanation: <plaintext> element renders everything following start tag as raw text, without interpreting any HTML. There is no need for closing tag since everything written after this tag is considered as raw text. <shadow> element is an obsolete part of web component which is used to shadow DOM objects. <spacer> element allows insertion of spaces in documents. <noembed> is an obsolete non-standard which provides fallback to the content of browser.