The HTML tag that is not valid or does not exist is the tag. Instead, HTML uses

for unordered lists,
for ordered lists, and
1. for list items. The tag is not an actual HTML tag. This is confirmed by a detailed explanation from various sources including Testbook and other HTML references.

   Also, an example of invalid tags includes any tags outside the standard ones such as , which does not exist in HTML (valid header tags range from

   to

   ).

   Therefore, if among given options or any tag not matching standard HTML tags appears, that would be the one that is not a valid HTML tag.