Choice "D" is correct. OSHA contains a "whistleblower" protection provision.
Choice "a" is incorrect. OSHA does not require a risk-free work environment; that would be impossible. Rather, OSHA regulations generally try to make workplaces as safe as they reasonably can be.
Choice "b" is incorrect. OSHA does not allow inspection at any time, for example, inspections generally must be during normal hours of operation.
Choice "c" is incorrect. OSHA allows states to adopt other safety regulations.