[Solution from source] With tert-butyl methyl ether, HI cleavage goes by SN1 through the stable tert-butyl carbocation, giving tert-butyl iodide and methyl alcohol. For the primary and secondary alkyl methyl ethers the reaction is SN2, where I⁻ attacks the less hindered methyl group, giving methyl iodide and the other alcohol. Answer (3).