The correct option is
A Nerve fibres with myelinated sheath
White matter is a component of the central nervous system, in the brain and spinal cord. The white matter is placed on the grey matter in the brain while it lies outer to the grey matter in the spinal chord. It consists mostly of glial cells and myelinated axons that transmit signals from one region of the cerebrum to another and between the cerebrum and lower brain centres.
Thus, the correct answer is option A.