Chemical formula of ammonium phosphate is:

The correct option is C $(N{H}_4{)}_{3}P{O}_4$
The first step is to write the symbol of the ions with the positive or negative charge upon it.

Ammonium ion is represented as $N{H}_4^{+}$ ion and phosphate ion is represented as $P{O}_4^3$${}^{-}$ ion.

The second steps involves interchanging the numbers indicating the charge of the ions.

Therefore for ammonium ion it will be 3 and for phosphate ion, it will be 1.

Now after simplifying to the smallest whole number , the chemical formula will be $(N{H}_4{)}_{3}P{O}_4$.