Good question! Citypopulation.de describes the area in question as Ruhrgebeit (literally, Ruhr Region) which includes several cities, including the two largest and well-known of Dortmund and Essen, and which has an aggregate population of 5.2 million. Ruhrgebeit is a region larger than Luxembourg that contains a swathe of cities and rural areas, and it doesn't exist as a city, so that name would be a nonsensical answer - but we still have a giant population centre to deal with that has to appear as an answer somehow. The best solution we've been able to come up with (in a situation with no perfect solution) with is to identify Dortmund and Essen as answers, which are both conurbations of well over a million each, and to also include cities close to them within the Ruhrgebeit as type-ins, and Ruhr as a type-in for them both. If you have any better solutions, please let us know!
My solution would be to choose a data source and stick with it. If you want to use citypopulation.de's "agglomerations", then use them for the whole quiz without alteration. Or if you want to alter it by partitioning the Ruhrgebiet, then say so in the instructions. And if you do choose to partition the Ruhrgebiet, check and see if whatever source says Dortmund and Essen are million+ conurbations doesn't say that Duisburg is one, too.