Answer : The correct option is, (A) The hydronium ion concentration in the solution.
Explanation :
Acid-base indicator : It is used in the titration. It is a substance which changes the color with the pH.
Acid-base indicator are the weak acid-base indicators which when dissolved in the water, it dissociate slightly and form the ions.
When the indicator dissolved in the water then it dissociate into
ion and then the color change of the pH indicator.
Let us consider an indicator which is a weak acid, with the formula HIn.
At equilibrium, the chemical equation of acid indicator will be,

Hence, the correct option is, (A) The hydronium ion concentration in the solution.