The Main Reason for the Observed Slowdown of Many Pulsars

The main reason for the observed slowdown of many pulsars is

A) friction between the stellar surface and the surrounding nebular material.
B) a slow buildup of the magnetic field; rotational energy is transferred to magnetic energy.
C) the loss of rotational energy to provide energy for the emission of radiation.
D) the slowing of rotation caused by slow expansion and redistribution of mass of the source, similar to the slowdown of a spinning skater.