Final answer:
The supervisor is responsible for ensuring that workers are properly using ladders, as they oversee workplace safety.
Step-by-step explanation:
The individual who must ensure that workers are properly using ladders is A) The supervisor. The supervisor is responsible for the health and safety of their employees in the workplace. This includes making sure that all equipment, including ladders, is used in a safe manner according to the Occupational Safety and Health Administration (OSHA) standards or relevant safety guidelines.