Final answer:
Acids release proton ions (H+) when dissolved in water. These protons are the result of hydrogen atoms losing their electrons. Strong acids completely ionize and release all their hydrogen ions, whereas weak acids only partially ionize.
Step-by-step explanation:
Acids release proton ions when in water. An atom of hydrogen, which acids release, is composed of one proton and one electron. When a hydrogen atom loses its electron and becomes an ion (H+), it is essentially just a proton, due to its lack of a neutron. Therefore, when acids dissolve in water they release hydrogen ions (H+), which are protons.
Acids are classified by the number of protons they can donate in reaction with water. Strong acids, such as hydrochloric acid (HCl), ionize completely in water, releasing all their hydrogen ions. Weak acids, like vinegar or acetic acid (CH3COOH), do not ionize completely and only release some of their hydrogen ions into the solution.
To summarize, when dissolved in water, the substance that acids release are H+ ions, or protons, making the correct answer A) Proton.