Answer is: A. covalent bonds.
Insulator most likely has covalent bonds, because it must have low electrical conductivity.
1) Substances with covalent bond not conduct an electric current in liquid and solid state, because they not have free ions or electrons.
2) Substances with ionic bond conduct an electric current in liquid, but not in solid state, because in liquids ions are mobile, contrary to solids where ions are fixed.
3) Substances with metallic bond conduct an electric current in liquid and solid state, because they have mobile electrons.
