The correct answer is - D. grassland.
All of the mentioned regions are examples of grasslands. Even though they differ in their characteristics, they have the different types of grasses both by type and height, different precipitation, different climate conditions, they still are occupied by grasses and fall into the same classification.
The steps southern Russia are mainly covered with low grasses, and the climate is continental dry with very hot dry summers and very cold dry winters.
The African veldt is covered with medium sized to high grasses and it receives rain only in one part of the year, it is constant summer.
The Argentinean pampas is covered with medium to high grasses, experiences seasons and small amount of rain.