Which light packs the highest energy per photon

It is the "Ultraviolet".  Ultraviolet means "beyond violet"  which comes from Latin ultra, "beyond" violet being the color of the highest frequencies of visible light. This light has a higher frequency than violet light. Of the visible spectrum, violet waves have the most energy. When the wavelength decreases, there is an increase in the frequency and so does the energy that the waves carry. Short wavelengths have the most energy. Waves that are red have a relatively long wavelength, which are 700 nm range, and violet waves are much shorter which are roughly half of that. Violet waves have the shortest wavelength of the visible light spectrum. They carry the most energy. The one which has the highest energy is violet. In the rainbow colors, it has the shortest wavelength, thus, it contains the highest energy.

They are all decomposition reactions.

The correct answer is c