Which band would be primarily filled with electrons in an insulator?

In an insulator, the valence band is primarily filled with electrons. The valence band represents the energy levels where electrons are bound to atoms and participate in bonding. In insulators, there is a significant energy gap between the valence band and the conduction band, which means that electrons in the valence band cannot easily move into the conduction band to conduct electricity. This high energy barrier prevents most of the electrons from becoming free charge carriers, which is why insulators do not conduct electricity well.

In contrast, the conduction band is typically empty in insulators because there are not enough electrons with sufficient energy to jump from the valence band into this higher energy state. The N-type and P-type bands refer to types of doping in semiconductors and are not applicable to insulators which do not have free charge carriers under normal conditions. Thus, in the context of the question, the valence band is the primary source of electrons for an insulator.