The correct answer is MOSCOW.
The population sizes of each of the cities are the following:
- St. Petersburg: second largest city in Russia with 5,351,935 inhabitants in 2018 and with a metropolitan area with a total population of 5,8 millon people.
- Moscow: is the main economic, political, cultural and scientific center in Eastern Europe and the capital of Russia. It is also the largest city in Europe, in the whole continent. It has got 13.2 million inhabitants within the city limitsand 17 million if taking into consideration its metropolitan area.
- Minsk: is the largest city and the capital of Belarus. It has got 1,974,819 inhabitants according to the census from 2017.
- Kiev: is the capital of Ukranie, with 2,887,974 inhabitants. It is the 7th most populous city in Europe.