Concept:Coordinate bonds are covalent bonds where one atom donates both shared electrons.Explanation:Aluminium chloride exists as a dimer, Al2Cl6, in the solid state.In this dimer, two chlorine atoms act as bridges between the two aluminium atoms.Each bridging chlorine donates a lone pair of electrons to an aluminium atom.This donation forms coordinate (dative) bonds, which help stabilise the dimer structure.Answer:A. Al2Cl6