Question:
Which employer did Franz Baader work for in Dec, 1991?

Context:
 Franz Baader works for University of Erlangen-Nuremberg from Jan, 1985 to Jan, 1989. 
 Franz Baader works for RWTH Aachen University from Jan, 1993 to Jan, 2002. 
 Franz Baader works for German Research Center for Artificial Intelligence from Jan, 1989 to Jan, 1993. 
 Franz Baader works for TU Dresden from Jan, 2002 to Dec, 2022.

Answer:
German Research Center for Artificial Intelligence