Which Functional Group Acts as an Acid When Placed in Solution

Which functional group acts as an acid when placed in solution?

A) -OH
B) -NH₂
C) -SH
D) -COOH
E) -PO₄ The carboxyl group is found in amino acids and will donate a hydrogen ion to a solution resulting in the functional groups becoming -COO^- and increasing the acidity of the solution.