Question

Myelin sheath covers
A. Muscle cells
B. Axon of neurons
C. Blood vessels
D. Osteocytes

Answer 1

Myelin is formed in the central nervous system i.e CNS - brain, spinal cord and optic nerve by glial cells called oligodendrocytes and in the peripheral nervous system i.e PNS by glial cells called Schwann cells.

Complete answer:
Myelin is a lipid-rich (fatty) substance that surrounds nerve cell axons to insulate them and increase the rate at which electrical impulses (called action potentials) are passed along the axon. The myelinated axon can be likened to an electrical wire with insulating material around it. However, unlike the plastic covering on an electrical wire, myelin does not form a single long sheath over the entire continuous length of the axon. Rather, each myelin sheath insulates the axon over a single long section and, in general, each axon comprises multiple long myelinated sections separated from each other by short myelin sheath-gaps where it lacks myelin called nodes of Ranvier.
This "insulating" role for myelin is essential for normal motor function, sensory function and cognition as demonstrated by the consequences of disorders that affect it, such as the genetically determined leukodystrophies; the acquired inflammatory demyelinating disorder, multiple sclerosis; and the inflammatory demyelinating peripheral neuropathies

Thus the correct option is B. Axon of neurons

Note:
Demyelination is the loss of the myelin sheath insulates the nerves, and is the hallmark of some neurodegenerative autoimmune diseases like multiple sclerosis, acute disseminated encephalomyelitis, neuromyelitis optica, transverse myelitis, chronic inflammatory demyelinating polyneuropathy, Guillain–Barré syndrome, central pontine myelinolysis, inherited demyelinating diseases such as leukodystrophy, and Charcot–Marie–Tooth disease.