The grasslands are vast areas that are covered with grasses. The type of grasses varies from place to place, and they can be short grasses, medium height, or tall grasses. The grasslands tend to be semi-arid, where the majority of the year is dry, and the majority of the rainfall occurs only in one period of the year. These vast grass covered areas are found deeper into the continents, and they are usually located between the forests and the deserts. As they go further into the interior, the grasslands gradually are replaced by deserts where the precipitation levels are below 25 cm annually.
Based of the map, you can infer that it's D because if you look the rivers are running through multiple countries and act as a border for the countries of Germany, Austria, Hungary, etc.
