Correct Option: D

An element which exists to facilitate generation of key material is <keygen>. <isindex> is an obsolete HTML element. It puts a text field in a page for querying the document. It is deprecated as of HTML 4.1, because same behavior can be achieved with HTML form. All major browsers have now removed <isindex>. <dir> contains list of directories. Those commands that user can invoke are represented by <command>.