The correct option is C Nerve fibre
Myelin sheath is a protective covering that surrounds fibres called axons (Nerve fibres). It is a fatty white substance and essential for the proper functioning of the nervous system. The main function of the myelin sheath is to increase the speed at which impulses propagate, in myelinated fibres, impulses hop or "propagate by saltatory conduction." So, the correct answer is option B.