The molecular formula of Aluminium (III) nitrate is:
(A)$Al\left( N{{O}_{3}} \right)$
(B)$Al{{\left( N{{O}_{2}} \right)}_{3}}$
(C)$A{{l}_{2}}{{\left( N{{O}_{3}} \right)}_{3}}$
(D)$Al{{\left( N{{O}_{3}} \right)}_{3}}$

Hint: Aluminium has 3 electrons in its valence shell and has a stable oxidation state of +3, and nitrate ion has a net negative charge of -1. Oppositely charged particles stick together to form an ionic compound. Their ratio must be so that there is no net charge on the compound.

Complete step by step answer: Aluminium nitrate is a salt of nitric acid that is white and soluble in water. Aluminium is abundantly found in the earth's crust combined with elements such as fluorine , silicon, and oxygen. Aluminium nitrate has a chemical formula $Al{{\left( N{{O}_{3}} \right)}_{3}}$. The other names of aluminium nitrate are nitric aluminium salt, aluminium (III) nitrate.
Aluminium has a stable oxidation number +3 and nitrate ion is $N{{O}_{3}}^{-}$, it has -1 oxidation state. Therefore, to fulfill the valency of aluminium, three nitrate ions will be attached with the one aluminium atom.
Aluminium nitrate appears to be quite valuable and has various uses. First of all, it is used as a powerful oxidizing agent. It is often used as a corrosion inhibitor, also used in petroleum refining, transformer core laminate and in uranium extraction. During combustion, it produces toxic nitrogen oxides.

Therefore the correct answer is (D) option.

Note: Aluminium nitrate is white crystalline solid. It has a melting point of ${{72.8}^{\circ }}C$ and boiling point of ${{135}^{\circ }}C$. It has molar mass of 212.996g/mol. Aluminium nitrate can be synthesized by reaction of barium nitrate and aluminium sulphate.