Final answer:
Among the given options, AlPO4 (aluminum phosphate) is known to be the least soluble in water due to its low solubility product, indicating that it forms very few ions in solution.
Step-by-step explanation:
The question is asking which compound among the given options is the least soluble in water. The solubility of a compound in water is greatly influenced by the compound's ionic character and the rule of like dissolves like. Typically, compounds that produce ions that react with water or form weak interactions will be less soluble. From the options given, AlPO4 (aluminum phosphate) is known to be the least soluble in water. This is because, unlike the other options, which form more soluble ions, the Al3+ and PO43- from AlPO4 have a very low solubility product (Ksp), indicating that very little of the compound dissolves to form these ions in water.