Why Do We Believe a Magnetar Can Have a Much

Why do we believe a magnetar can have a much stronger magnetic field than that of an ordinary neutron star?

A) Magnetars were formed from supergiant stars many times larger than the stars that produced ordinary neutron stars.
B) When first formed as neutron stars, magnetars were spinning rapidly enough that the magnetic fields produced by convection became joined.
C) Magnetars are superconducting throughout.
D) Magnetars have a higher proportion of protons and thus have stronger electric currents.