Which of the following is responsible for the characteristic whitish colour of the neural tissue?

It is the insulating envelope of myelin that surrounds the core of a nerve fiber or axon and that facilitates the transmission of nerve impulses formed from the cell membrane of the schwann cell in the peripheral nervous system, and from oligodendroglia cells. If the axons have myelin sheaths then the pathway appears bright white because myelin is primarily lipid. If nearly  all of the axons lack myelin sheaths or are unmyelinated then the pathway will appear a darker beige colour, which is generally called grey. It is the myelin sheath that gives the whitish colour to the areas of white matter.