Final answer:
Workers' compensation is regulated primarily by the states, where employers contribute to state-level funds that provide benefits to injured workers.
Step-by-step explanation:
Workers' compensation is a form of insurance providing wage replacement and medical benefits to employees injured in the course of employment. According to historical progress in labor laws, employers are required by law to pay a small percentage of the salaries they pay into funds, which are typically run at the state level. These funds pay benefits to workers who suffer an injury on the job. Therefore, workers' compensation is regulated primarily by B. States.