Concept:An acid in water produces hydrogen ions, making the solution acidic.
Explanation:H3PO4 is phosphoric acid, a weak triprotic acid.
In aqueous solution, it ionises slightly and releases
H+ ions, so the solution has a pH below 7.
NH4Cl is an acidic salt because its ammonium ion hydrolyses, but
H3PO4 is the actual acidic compound.
Na2CO3 forms an alkaline solution, and
MgCl2 is treated as a neutral salt in this context.
Therefore, the clearest example of a compound that is acidic in solution is phosphoric acid.
Answer:A.
H3PO4