The correct answer is - A. The Baltic Sea.
The second biggest Russian city, St. Petersburg, is located on the shores of the Baltic Sea, more specifically, on the westernmost part of the Baltic Sea, where the sea gets the furthest inland and creates a bay-like shape. It is great importance for the city that comes out on this sea because through it, it is connected with the like of Finland, the Scandinavian countries, as well the Baltic countries, Poland and Germany, and this is of great importance for the economy of this great city.