Concept:Acids are proton donors that react with active metals to release hydrogen gas.Explanation:Acids have a pH below 7, taste sour, and turn blue litmus paper red.Options A, B, and C describe properties of bases instead of acids.A unique acid property is the reaction with active metals like zinc or magnesium to produce hydrogen gas.For example: Zn+2HCl→ZnCl2+H2↑This test helps distinguish acids from bases.Answer:D. React with metals to produce hydrogen gas.