Conspicuous annual rings are formed in temperate deciduous plants.

Explanation:

1. Annual Rings:
Annual rings are concentric circles formed in the trunk of a tree or woody plant. They are composed of two types of cells - the springwood and the summerwood. The springwood is formed during the spring season when the tree experiences rapid growth, while the summerwood is formed during the summer season when growth slows down. These rings can be observed when a tree is cut or when a tree's cross-section is examined.

2. Formation of Annual Rings:
The formation of annual rings is influenced by the climate and growth patterns of the tree. In temperate deciduous plants, the growth of the tree is influenced by the changing seasons. During the spring, when the temperature and moisture levels are favorable, the trees experience rapid growth. This leads to the formation of wider springwood rings. In contrast, during the summer, when the conditions are less favorable, the growth slows down, resulting in narrower summerwood rings.

3. Conspicuous Nature:
The annual rings in temperate deciduous plants are particularly conspicuous due to their distinct contrast in color and width. The springwood is usually lighter in color and wider, while the summerwood is darker and narrower. This contrast is often visible to the naked eye and can be used to determine the age of the tree. By counting the number of rings, one can estimate the age of the tree, as each ring represents one year of growth.

4. Tropical Evergreen Plants:
Tropical evergreen plants, such as those found in rainforests, do not experience distinct seasons like temperate regions. The climate in tropical regions remains relatively constant throughout the year, with no significant variations in temperature or moisture levels. As a result, the growth of these plants is continuous, and the formation of distinct annual rings is not as pronounced.

Conclusion:
In conclusion, the most conspicuous annual rings are formed in temperate deciduous plants. These plants experience distinct seasonal changes, leading to the formation of contrasting springwood and summerwood rings. On the other hand, tropical evergreen plants do not exhibit such pronounced annual ring formation due to their consistent climatic conditions.

D, as there is no environmental variation.