H⁺
Explanation:
There are three definitions of acids: Arrhenius, Bronsted - Lowry, and Lewis.
An Arrhenius acid is a substance that when dissolved in water will release a proton (H⁺ or hdyronoum, H₃O⁺) in solution.
The definition of Bronsted-Lowry is not limited to aqueous solution: an acid is a substance that releases protons in any solvent. So it includes, the Arrhenius acids but also other acids.
The Lewis Acid definition is wider. It includes both Arrhenius and Bronsted-Lowry acids and other substances that do not release protons. A Lewis acid is a substance that accepts an electron pair.
Thus, when an acid is dissolved in a solution, following Bronsted-Lowry definition, H⁺ ions are formed.