Which one of the following reactions does not involve either oxidation or reduction ?

$$\begin{gathered} \stackrel{*}{\mathrm{Cr}}{\mathrm{O}}_4^{2-} \stackrel{*}{{\mathrm{Cr}}_2}{\mathrm{O}}_7^{2-} \\ \mathrm{x} + \left[\left(-2\right) \times 4\right] = -2 ; 2\mathrm{x} + \left(-2\right) \times 7 = -2 ; \\ \mathrm{x}= + 6 ; 2\mathrm{x} = 14 - 2 = 12 ; \\ \mathrm{x} = \frac{12}{2} = +6 ; \end{gathered}$$

In this reaction neither oxidation nor reduction are not involved.  there is no change in oxidation number.