Explanation:
The correct answer is option 'D' - tropical thorn forest.

Tropical thorn forest:
The tropical thorn forest is characterized by dry and arid conditions, with a scarcity of water and high temperatures. These conditions make it difficult for plants to survive, resulting in a sparse vegetation cover.

Leafless plants:
In order to adapt to the harsh environment of the tropical thorn forest, many plants have evolved to be leafless for most of the year. This adaptation helps them conserve water and reduce the risk of desiccation.

Reduced transpiration:
By being leafless, these plants reduce the surface area exposed to the sun and wind, which helps minimize water loss through transpiration. Transpiration is the process by which plants lose water through their leaves. In dry and arid conditions, high rates of transpiration can be detrimental to plant survival.

Modified stems:
Leafless plants in the tropical thorn forest often have modified stems that serve multiple functions. These modified stems can store water, provide protection against herbivores, and perform photosynthesis. The ability to store water in the stems helps these plants survive during extended periods without rainfall.

Examples of leafless plants:
Some examples of leafless plants found in tropical thorn forests include cacti, succulents, and certain species of acacia. These plants have adapted to the harsh conditions by developing specialized structures such as spines, thorns, and water-storing tissues.

Conclusion:
In conclusion, plants in the tropical thorn forest remain leafless for most of the year as an adaptation to the dry and arid conditions. Being leafless helps them conserve water and reduce the risk of desiccation. These plants have evolved modified stems that serve multiple functions and allow them to survive in this challenging environment.