Updated to replace 4 cities where skyscrapers have now been built: Cairo, Chennai, Baghdad and Pune. Cairo's new skyscraper is located in the new capital that is under construction on the outskirts of the city, which is considered to be part of the Cairo metro area for this quiz.
The source still has Berlin's population at 4.7 million, plus a skyscraper (Estrel Tower) has just been topped out this year. The Cologne-Bonn agglomeration has indeed now been merged with the Ruhr in citypopulation.de, which means I should probably remove that answer. I'll re-check the rest of the answers as well and update the quiz accordingly.
Bonn, a city with the only german skyscraper outside of Frankfurt, is part of the Rhein-Ruhr.