Final answer:
Among the phosphate compounds listed, only (NH4)3PO4 (ammonium phosphate) is soluble in water, mainly due to the presence of the NH4+ ion which typically forms soluble salts.
Step-by-step explanation:
Out of the list of phosphate compounds provided, (NH4)3PO4 (ammonium phosphate) is soluble in water. This is because compounds containing the ammonium ion (NH4+) are typically soluble in water. On the other hand, most carbonates, phosphates, and sulfides are sparingly soluble, with some exceptions such as salts that contain Group IA, IIA, or NH4+ ions.
In the case of compounds like Mg3(PO4)2, Ca3(PO4)2, AlPO4, and Ag3PO4, these are generally insoluble in water due to their ionic composition and low solubility product constants (Ksp).