In aqueous solution proton exists as ________ (acid/base).

Hint: A solution in which the solvent is water is called an aqueous solution. In an aqueous solution, the solute or dissolved ions or molecules are surrounded by the water molecules and make many bonds within the water. The dissolved ions or molecules can spread throughout the water. The reactions which occur in aqueous solution are mostly double-displacement or metathesis reactions i.e. when a cation is displaced to form an ionic bond with the other anion. The cation which is bonded with the latter anion will dissociate and make a bond with the other anion. The aqueous solution which conducts strong electric current contains strong electrolytes and the aqueous solution which conducts poor electric current contains weak electrolytes. The strong electrolytes get completely dissociated in an aqueous solution while the weak electrolytes do not completely dissociate in an aqueous solution.

Complete step by step answer:
The hydrogen ion in an aqueous solution is no more than a proton, bare nucleus. Since it carries a single unit positive charge this charge is concentrated into a volume of space which is only about a hundred-millionth as large as the volume occupied by the smallest atom (for example think of a pebble sitting in the middle of a sports stadium). In aqueous solution the proton makes a coordinate bond which creates the hydronium ion $\left( {{H_3}{O^ + }} \right)$.

$H{}_2O + {H^ + } \to {H_3}{O^ + }$

So the hydronium ion is acting as a conjugate acid and the proton in the aqueous solutions acts as an acid.

So, the correct answer is Option A .

Note: Acid is a hydrogen-containing substance that is capable of donating a proton or hydrogen ion to another substance. Acids turn blue litmus red. If the pH of a solution is less than 7 then the solution will be acidic. The base is a molecule or ion which can accept the hydrogen ion or proton from an acid. Bases turn red litmus blue. If the pH of a solution is greater than 7 then the solution will be basic.