In LaTeX-mode, there is
latex-close-block, bound to C-c C-e.
Is there a function to programmatically close an XML tag in nXML-mode?
You can close an XML block in nXML-mode via
To find such functions, have a look at the mode's description, accessible via C-h m when in nXML-mode. It says
C-c / finishes the current element by inserting an end-tag.