The correct answer is d) self-employed persons.
The Occupational Safety and Health Act (OSHA) was created to ensure safe and healthy working conditions for employees. However, it primarily covers employees who are under the jurisdiction of an employer. Self-employed individuals do not have the same employer-employee relationship that OSHA aims to protect, thus they are not covered by the act.
Federal agents, hospital nurses, and crane operators are all employees who work under employers and are therefore entitled to OSHA protections. This is why self-employed persons are the exception in this context.