The correct answer is - b. Mainly humid-continental with cool summers.
Most of Central and Eastern Europe are dominated by the continental climate, though it has to be pointed out that the amount of rainfall is bigger than what it is the standard for the continental climate types, thus it is humid continental.
The reason why the climate is of this type in this regions is that it is mostly under the influence of the continental air masses, but since Europe's coastline often gets deeper into the mainland, there's also influence from the sea, which results in higher precipitation than usual.
The summers are usually dry but not very hot, while the winters are snowy and with pretty low temperatures. The spring and autumn are the most rainy periods, with having milder temperatures.