The molecular formula of aluminium sulphate is: 

The chemical structure of an organic compound indicates the true formula of its one molecule by displaying the exact number of atoms, kind of bond (saturated or unsaturated), and functional group present in one molecule of the compound. The valance (outermost) shell of the aluminium atom contains three valence electrons. Therefore, by sharing its three valence electrons, this atom can only form three bonds. The molecule of sulphur in which it is bound by two double bonds to two oxygen atoms is known as sulphur dioxide. With two oxygen atoms as a bonding partner, sulphur still has two valence electrons in its valence shell after using the four electrons from its six-electron valence shell. Therefore, the three sulphur dioxide molecules and two aluminium atoms can bind.
Equal numbers of charges are shared by the cation and anion during the creation of an ionic bond.

The sulphate ion is a polyatomic anion of sulphur and oxygen. It has a charge of -2. Now aluminium ion has a charge +3. To neutralize the charge of the molecule as a whole; they combine in such a way that, the number of sulfate ion present is three and the number of aluminium is two. Therefore, the formula becomes $A{l}_2{\left(S{O}_4\right)}_3$.

Hence, option D is correct.