Which external component can be used for defense?

Horns serve as a notable external component that can be utilized for defense in various livestock species. They are hard, bony projections that can provide physical protection against predators. In many animals, horns are primarily used not just for defense but also play a role in establishing dominance during mating rituals and in social interactions. Their strength and size can deter potential threats, making them effective in safeguarding the animal.

In contrast, while bones provide structural support and protection for internal organs, they are not considered external components that can actively engage in defense. Hair, on the other hand, serves mainly as insulation or a barrier against environmental factors but does not provide a significant means of defense. Dewclaws are small, non-weight-bearing structures that do not contribute significantly to defensive behaviors. Thus, horns are the most effective external component for defense among the given choices.