Final answer:
A guardrail should be capable of withstanding a force of at least 250 pounds-force (option D) to provide adequate protection.
Step-by-step explanation:
A guardrail should be capable of withstanding a force of at least 250 pounds-force (option D). This is because a guardrail needs to be able to withstand the impact of a moving vehicle or a person falling against it, which can generate a significant amount of force. By setting the minimum requirement at 250 pounds-force, the guardrail can provide adequate protection in most scenarios.