(d) is correct. Temperate region and tropical region

The correct answer is option 'D', which categorizes the world's grasslands into Temperate regions and Tropical regions. Let's understand why this is the correct answer and explore the characteristics of each category.

1. Temperate regions:
- Location: Temperate grasslands are found in the mid-latitudes, between the polar regions and the tropics, typically between 30° and 55° latitude in both the northern and southern hemispheres.
- Climate: These regions have a moderate climate with distinct seasons. Summers are warm to hot, while winters can be cold with freezing temperatures.
- Vegetation: The dominant vegetation in temperate grasslands consists of grasses, herbs, and forbs. Trees are usually absent or scattered in these regions.
- Examples: The North American Great Plains, the Eurasian steppes, and the Pampas of South America are some well-known examples of temperate grasslands.

2. Tropical regions:
- Location: Tropical grasslands are located near the tropics, between the Tropic of Cancer and the Tropic of Capricorn.
- Climate: These regions have a tropical climate characterized by high temperatures throughout the year. They experience wet and dry seasons rather than distinct summer and winter seasons.
- Vegetation: Tropical grasslands are dominated by tall grasses, with scattered trees and shrubs. The grasses are usually adapted to withstand drought and fire.
- Examples: The African savannas, the llanos of South America, and the Australian savannas are notable examples of tropical grasslands.

By categorizing the world's grasslands into Temperate and Tropical regions, we can better understand and study their unique characteristics, including climate, vegetation, and geographical distribution. This classification helps in analyzing the different ecological processes and adaptations that occur in these grassland ecosystems.

It is important to note that the other options provided (a, b, and c) do not accurately categorize the world's grasslands. Option 'a' incorrectly combines Thorny and Deciduous regions, while option 'b' reverses the order of the categories. Option 'c' incorrectly combines Thorny and Tropical regions. Therefore, option 'D' is the correct answer as it accurately categorizes the grasslands into Temperate and Tropical regions.