answer e) is correct. During active strech excitation of gamma efferent neurons of anterior grey column of spinal cord produces contraction of polar regions of the intra fusal fibres and there by not contractile equatoial regions of nuclear bag and chain fibres are actively streched. this results in excitation of alpha motor neurons through the monosynaptic loop of strech reflex conscequently the extra fusal fibres of the corresponding muscle are contracted until it shortens to equal with the degree of contraction of muscle spindle. this method of contraction of skeletal muscle maintains its residual length. The alpha and gamma motor neurons are excited simaltaneously by the pyramidal tract, the alpha neurons being the fast conductors initiate muscle contraction with out limit of shortening and the gamma neurons through the reflex loop maintain muscle contraction with desired length.