Final answer:
Toe boards on a scaffold must extend a minimum of 3.5 inches above the platform, which is not listed among the multiple-choice options provided.
Step-by-step explanation:
When toe boards are required on a scaffold, they must extend at least 3.5 inches (approximately 9 cm) above the platform. This requirement is in place to prevent tools, materials or equipment from accidentally being kicked or pushed off the scaffold platform, posing a danger to workers or passersby below. However, the options provided in the question (2', 3', 4', 5') do not match the standard regulation. The correct answer, which falls outside of the provided options, would be that toe boards should extend a minimum of 3.5 inches above the platform, ensuring a safe working environment.