Question:
Who was the head of Vienna in Apr, 1991?

Context:
 Josef Neumayer is the head of the government of Vienna from May, 1910 to Dec, 1912. 
 Hermann Neubacher is the head of the government of Vienna from Mar, 1938 to Dec, 1940. 
 Richard Schmitz is the head of the government of Vienna from Apr, 1934 to Mar, 1938. 
 Michael Häupl is the head of the government of Vienna from Nov, 1994 to May, 2018. 
 Richard Weiskirchner is the head of the government of Vienna from Dec, 1912 to May, 1919. 
 Felix Slavik is the head of the government of Vienna from Dec, 1970 to Jul, 1973. 
 Franz Jonas is the head of the government of Vienna from Jun, 1951 to Jun, 1965. 
 Karl Seitz is the head of the government of Vienna from Nov, 1923 to Feb, 1934. 
 Philipp Wilhelm Jung is the head of the government of Vienna from Dec, 1940 to Dec, 1943. 
 Helmut Zilk is the head of the government of Vienna from Sep, 1984 to Nov, 1994. 
 Karl Lueger is the head of the government of Vienna from Apr, 1897 to Mar, 1910. 
 Bruno Marek is the head of the government of Vienna from Jun, 1965 to Dec, 1970. 
 Theodor Körner is the head of the government of Vienna from Apr, 1945 to Jun, 1951. 
 Hanns Blaschke is the head of the government of Vienna from Dec, 1943 to Apr, 1945. 
 Jakob Reumann is the head of the government of Vienna from May, 1919 to Nov, 1923. 
 Leopold Gratz is the head of the government of Vienna from Jul, 1973 to Sep, 1984. 
 Michael Ludwig is the head of the government of Vienna from May, 2018 to Dec, 2022.

Answer:
Helmut Zilk