A student has three unknown solutions (A,B, and C) with pH values of 5, 10 & 12.5 respectively. The solution with maximum hydroxide ion concentration is

pH is a number that represents whether a solution is acidic or not.

It is represented as-

$$\begin{gathered} \mathrm{pH} = -\log \left[{\mathrm{H}}^{+}\right] \\ \mathrm{or} \\ \mathrm{pH} = - \log \left[{\mathrm{H}}_3{\mathrm{O}}^{+}\right] \end{gathered}$$

A very high hydrogen (${\mathrm{H}}^{+}$) concentration is represented by a low pH indicating that the solution is highly acidic in nature.

On the other hand, high pH means lower hydrogen ion (${\mathrm{H}}^{+}$) concentration and higher hydroxide ion (${\mathrm{OH}}^{-}$) concentration.

As solution C has the highest pH, that means it has the highest hydroxide ion concentration among the given solutions.

Hence, the correct option is (C).