Which material is specified for vertical supports of guardrails?

Answer: B — Steel

Steel is specified for vertical supports of guardrails due to its strength and durability. It is capable of withstanding significant forces, making it essential for maintaining safety in various conditions, especially in high-traffic areas. Steel's properties allow it to support the weight of the guardrails effectively while also resisting impacts from vehicles. This choice ensures that the guardrails remain structurally sound and functional over time, providing the necessary protection for both drivers and pedestrians. Other materials, while they may have certain applications, do not offer the same level of resilience and longevity required for guardrail vertical supports, especially in environments subjected to severe stress and wear.