Explanation:

The correct answer is option 'B': Bark.

Definition of Bark:
The bark is the protective outer covering of the trunk, branches, and roots of a tree. It is made up of several layers that serve to protect the tree from various environmental factors such as pests, diseases, extreme temperatures, and mechanical damage. The bark also helps to transport nutrients and water throughout the tree.

Importance of Bark:
1. Protection: The bark acts as a protective barrier for the tree, shielding it from physical damage caused by animals, insects, and weather conditions. It prevents the underlying tissues from drying out and keeps them safe from external threats.

2. Insulation: The bark provides insulation to the tree. It helps regulate the temperature of the tree, preventing it from getting too hot or too cold. This is especially important during extreme weather conditions.

3. Transport of Nutrients: The innermost layer of the bark, known as the phloem, is responsible for transporting sugars, nutrients, and hormones from the leaves to other parts of the tree. This ensures the proper growth and development of the tree.

4. Disease Resistance: The bark contains chemicals and compounds that help protect the tree from diseases and pests. It acts as a natural defense mechanism, preventing the entry of harmful organisms into the tree's tissues.

Other Options:
- Stem: The stem is the main structural part of the plant that supports the branches, leaves, flowers, and fruits. While the trunk is a type of stem, it specifically refers to the main stem of a tree. It is not the correct answer in this case.

- Pigment: Pigments are substances that give color to plants and animals. While some tree barks may contain pigments that give them a specific color, it is not the primary function of the bark. Therefore, it is not the correct answer in this case.

- Roots: Roots are the underground part of the tree that anchors it in the soil and absorbs water and nutrients. While roots play a crucial role in the overall health and stability of the tree, they do not directly protect the trunk. Therefore, roots are not the correct answer in this case.

In conclusion, the bark of a tree is responsible for protecting the trunk and other parts of the tree by acting as a physical barrier and providing insulation, nutrient transport, and disease resistance.