Which of the following protects the trunk of a tree from insects, disease, and injury?

Bark is the outer layer of a tree that serves as a protective barrier for the trunk. It acts as the first line of defense against various threats, including insects that might bore into the wood, diseases that could infiltrate through openings, and physical injuries that could compromise the integrity of the tree. The structure of bark helps safeguard the inner tissues from environmental stresses and pathogens.

In contrast, xylem is responsible for transporting water and nutrients from the roots to the leaves, while phloem is responsible for the transport of sugars and other metabolic products downward from the leaves. The cambium is the layer of growth tissue that lies between the phloem and xylem, playing a crucial role in the growth of the trunk by producing new xylem and phloem cells. However, none of these inner layers serve the primary protective function that bark does, making it the correct answer in this context.