For the chemical formula of Ammonium Sulfate.

• The ammonium ion is $\text{NH}_4^+$.
• The sulfate ion is $\text{SO}_4^{2-}$.

To balance the $+1$ charge of the ammonium ion and the $-2$ charge of the sulfate ion, two ammonium ions are required for every one sulfate ion.

Combining two $\text{NH}_4^+$ ions with one $\text{SO}_4^{2-}$ ion gives the formula $(\text{NH}_4)_2\text{SO}_4$.

(A) $(\text{NH}_3)_2\text{SO}_4$ - Incorrect, $\text{NH}_3$ is ammonia, not ammonium.
(B) $\text{NH}_3\text{SO}_4$ - Incorrect.
(C) $\text{NH}_4\text{SO}_4$ - Incorrect, charges are not balanced.
(D) $(\text{NH}_4)_2\text{SO}_4$ - Correct.

The correct option is (D).