The correct answer is - Russian Plain.
The Eastern European Plain, also known as the Russian Plain, is composed of vast lowlands that are stretching from Poland to the Ural Mountains, thus to the Asian border.
This plain is lying on the territories of eastern Poland, the Baltic countries, Belarus, Ukraine, Moldova, Russia, and western Kazakhstan. Most of this area is widely used for agriculture, with the wheat being the primary crop in this region, and it also known as a region that has a very nice fertile soil.
The reason why the Eastern European Plain is also referred to as the Russian Plain is simple, because most of it is located on the territory of Russia.