Answer is: An acid is a proton donor. According to Bronsted-Lowry theory acid are donor of protons and bases are acceptors of protons (the hydrogen cation or H⁺). For example hydrochloric acid (HCl) in reaction with water (H₂O) give one proton to water and become chloride anion (conjugate base Cl⁻).
An acid is a proton donor, so choice (A) is the most correct. That is, they will release H+ ions when placed into water, or when placed into some sort of solution. This will allow the acid to dissociate into its constituent elements.
