Final answer:
H₃PO₄ (phosphoric acid) is considered an acid among the given options.
Step-by-step explanation:
The compound that is considered an acid among the given options is H₃PO₄ (phosphoric acid). Acids are substances that donate protons (H⁺ ions) when dissolved in water. Phosphoric acid is a triprotic acid that can donate three protons, and it is commonly used in soft drinks and fertilizers. Acids are characterized by their ability to donate hydrogen ions (H+) when dissolved in water, forming an aqueous solution. This definition is in line with the Arrhenius theory of acids and bases.