Right option is (b) <kbd>
Easy explanation: <mark> element is for highlighting the text/paragraph. <kbd> element is HTML Keyboard Input Element. It represents a span of inline text denoting textual user input from voice input, a keyboard or any other text entry device. By convention user defaults to rendering the contents of <kbd> element using its default monospace font. Defining instance of a term in HTML is represented by <dfn> element. <em> is same like <i></i>. It is emphasized text.
