Which of the following substances can be classified as an acid, and why does it belong in this category?

Of the options, hydrochloric acid (HCl) qualifies as an acid because it donates a proton (H⁺) when dissolved in water. The presence of a freely dissociable hydrogen ion is the defining feature of acids in the Brønsted–Lowry sense. When HCl dissolves, it splits into H⁺ and Cl⁻, and the H⁺ ions give the solution its characteristic sour taste and ability to turn red litmus blue. For example, a drop of HCl turns a pH‑indicator paper from red to blue, confirming its acidic nature. Thus, HCl is the clear acid among the choices.