Which of the following is an example of a physical structure that provides mechanical protection to plants?

Thorns serve as a physical structure that provides mechanical protection to plants. They are modified stems or branches that deter herbivores and can help protect the plant from physical damage. Their presence makes it more challenging for animals to feed on the plant, thereby reducing herbivory and enhancing the plant's chances for survival and reproduction.

In contrast, vascular tissue is involved in the transport of water, nutrients, and sugars throughout the plant, but it does not serve a protective role. Chlorophyll is a pigment crucial for photosynthesis, enabling plants to absorb sunlight and convert it to energy, but it does not provide any physical protection. Root hairs are extensions of root cells that increase surface area for water and nutrient absorption; while essential for plant health, they do not offer mechanical protection. Thus, thorns are specifically designed structures that fulfill the role of defense against physical threats.