Introduction:
Cactus plants are well-known for their ability to survive in harsh and arid environments. They have adapted to live in regions with very little water and high temperatures. As a result, cactus plants are commonly found in deserts around the world.

Desert Environment:
Deserts are characterized by extremely dry conditions, receiving very little rainfall throughout the year. The lack of water makes it difficult for most plants to survive in these regions. However, cactus plants have evolved to thrive in such environments.

Adaptations of Cactus Plants:
Cactus plants have several adaptations that enable them to survive in deserts:

1. Succulent Stem: Cacti store water in their thick and fleshy stems. This allows them to retain water for long periods and withstand drought conditions.

2. Reduced Leaves: Cactus plants have modified their leaves into spines or scales to minimize water loss through transpiration. This adaptation helps them conserve water and reduce the risk of dehydration.

3. Shallow and Widespread Root System: Cacti have shallow roots that spread out horizontally near the surface of the soil. This allows them to quickly absorb any available water, even from light rainfall or morning dew.

4. Waxy Outer Layer: The outer layer of cactus plants is covered with a thick, waxy substance called cuticle. This cuticle helps to prevent water loss through evaporation.

5. Camouflage: Some cacti have a greenish-gray coloration that helps them blend in with the surrounding desert environment. This camouflage helps protect them from predators and reduces the amount of sunlight they absorb, minimizing water loss.

Conclusion:
In conclusion, cactus plants are primarily found in deserts due to their remarkable adaptations to survive in arid conditions. Their succulent stems, reduced leaves, shallow root systems, waxy outer layer, and camouflage allow them to thrive in regions with little water and high temperatures. Therefore, the correct answer to the question is option 'D' - deserts.