To test the theory of using spectroscopy to measure the amount of the solution that has reacted, you would need to use a solution that has a color change or absorbs light at a specific wavelength upon reacting. This is because spectroscopy involves measuring the absorption or emission of light by a substance at different wavelengths, and a change in the substance's chemical composition can alter this property.
So, if any of the solutions in the three tubes produce a color change or absorb light at a specific wavelength upon reacting, then that solution can be used to test the theory using spectroscopy.