From the Wikipedia entry for SGML:
While HTML was developed partially independently and in parallel with
SGML, its creator, Tim Berners-Lee, intended it to be an application
of SGML.[citation needed] The design of HTML (Hyper Text Markup
Language) was therefore inspired by SGML tagging, but, since no clear
expansion and parsing guidelines were established, most actual HTML
documents are not valid SGML documents.
Later, HTML was reformulated
(version 2.0) to be more of an SGML application, however, the HTML
markup language has many legacy- and exception- handling features that
differ from SGML's requirements.
HTML 4 is an SGML application that
fully conforms to ISO 8879 – SGML.[14]
So the answer is that while it was first implemented using SGML-type markup, but it was in parallel rather than as a direct decendant of SGML.
Further, on the same article, the World Wide Web Consortium states:
The charter for the 2006 revival of the World Wide Web Consortium HTML Working Group says, "the Group will not assume that an SGML parser is used for 'classic HTML'".[15] Although HTML syntax closely resembles SGML syntax with the default reference concrete syntax, HTML5 abandons any attempt to define HTML as an SGML application, explicitly defining its own parsing rules,[16] which more closely match existing implementations and documents. It does, however, define an alternative XHTML serialization, which conforms to XML and therefore to SGML as well.
(my emphesis)
Therefore the answer is clearly "No - none of the above". XML can be classed as a SGML markup but HTML 1 is not, HTML 2 is, HTML 3 and 4 are not and HTML 5 uses it's own markup.
Time spent reading Wikipedia: 3 minutes.
