What determines pH of the solution?
A. Ratio of hydronium to hydroxide ions
B. Indicators
C. Amount of acid
D. Concentration

Hint :The concentration of hydrogen $ \left( {{H^ + }} \right) $ ions in a solution is measured by its pH, which is a measure of acidity. We know that the pH of water is a measurement of how acidic or basic it is. The range is 0 to 14, with 7 being the neutral value. Acidity is indicated by a pH less than 7, whereas a pH greater than 7 indicates a base. pH is a measurement of the proportion of free hydrogen and hydroxyl ions in water.

Complete Step By Step Answer:
When dealing with chemical reactions that take place in aqueous solutions, the hydronium ion is crucial. Its concentration in relation to hydroxide provides a clear indication of a solution's pH. As $ {H^ + } $ ions form, they form $ {H_3}{O^ + } $ by bonding with $ {H_2}O $ molecules in the solution. From this we can understand that ratio of hydronium to hydroxide ions determines the pH of the solution.
Hence, option A is correct.

Note :
Remember that the presence of hydronium ion in relation to hydroxide determines the pH of a solution. It determines whether a solution is acidic or alkaline. A shift in the number of H+ ions to the point that they outnumber the $ O{H^ - } $ ions results in a lower pH and an acidic solution. The rise in pH will result from a change in the number of $ {H^ + } $ ions to where they were less than the $ O{H^ - } $ ions.