Which of the following has the lowest melting point?

aluminum bromide has the lowest melting point

Aluminium bromide has a melting point of 97.5 °C when it is anhydrous and 93 °C when it is  hexahydrate. Aluminium bromide has a 252 °C hexahydrate boiling point and a 255 °C anhydrous boiling point.

Because aluminium [tri]bromide ${\mathrm{AlBr}}_3$ is a molecular compound with weak intermolecular interactions its melting point is lower

${\mathrm{AlI}}_3$ and ${\mathrm{AlBr}}_3$ are predominantly covalent compounds. Hence, ${\mathrm{AlI}}_3$  has higher M.P. than ${\mathrm{AlBr}}_3$ (London dispersion forces).

Hence correct option is D