/usr/lib/mono/4.5/Commons.Xml.Relaxng.dll is in libmono-relaxng4.0-cil 3.2.8+dfsg-4ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/mono/4.5/../gac/Commons.Xml.Relaxng/4.0.0.0__0738eb9f132ed756/Commons.Xml.Relaxng.dll