Which of the following has non – myelinated nerve fiber? 

• Myelin sheaths are a protective covering that surrounds axons, which are fibers. Our nerve cells are sleeved or covered in fatty tissue called myelin sheaths. 
• The central nervous system of our body contains these cells. The myelin sheath functions as an insulator to stop the neurons from short-circuiting.
• Non-myelinated fibers, or autonomic nerves, conduct nerve impulses slowly yet smoothly because they lack myelin covering. 
• The autonomic nervous system is a component of the peripheral nervous system that regulates involuntary physiologic processes including heart rate, blood pressure, respiration, digestion, and sexual arousal.

Hence, option D is the correct answer.