Introduction:
The trunk of a tree is the main part of the tree that extends from the ground to the upper branches. It serves as the support structure for the tree and is responsible for transporting water, nutrients, and sugars between the roots and leaves. The trunk is protected by a layer called the bark, which plays a vital role in the tree's growth, development, and protection.

Explanation:
The correct answer is option 'B' - Bark. Let's understand why the bark of the tree protects the trunk:

Bark as a Protective Layer:
1. The bark is the outermost layer of the tree trunk and serves as a protective covering. It shields the inner tissues of the trunk from external factors such as extreme temperatures, physical damage, pathogens, and insects.
2. Bark acts as a barrier against mechanical injuries like cuts, abrasions, and impacts. It helps prevent damage to the delicate living tissues beneath it.
3. The bark also acts as a defense mechanism against harmful pathogens and insects. It contains chemical compounds and secondary metabolites that deter pests and prevent the entry of disease-causing organisms.
4. In some tree species, the bark may have a rough or textured surface, which can deter climbing animals or insects from reaching the trunk and causing harm.

Bark and Growth:
1. The outermost layer of the bark is called the epidermis. It is responsible for protecting the underlying tissues and preventing water loss through evaporation.
2. Just beneath the epidermis is the phloem, a vascular tissue that transports sugars produced in the leaves to other parts of the tree. The phloem is essential for the tree's growth and development.
3. The innermost layer of the bark is the cambium, a layer of actively dividing cells. It is responsible for the growth of new bark and wood tissues.
4. As the tree grows, the bark expands and develops fissures or cracks. This allows the trunk to accommodate the increase in girth without restricting the growth of the tree.

Bark and Identification:
1. The bark of different tree species can vary in color, texture, and pattern. It can be smooth, rough, thick, thin, or even peeling.
2. These unique characteristics of the bark can help in identifying and distinguishing between different tree species.
3. Some tree barks have distinctive features such as deep furrows or corky ridges, which aid in species identification.

Conclusion:
The bark of a tree serves as a protective layer for the trunk. It defends against external factors and provides a physical barrier to prevent damage to the delicate inner tissues. Additionally, the bark plays a crucial role in the growth and development of the tree by transporting sugars, accommodating growth, and providing unique characteristics for species identification.