Final answer:
The correct unit for the energy of a photon emitted when an electron transitions from an excited state is (b) Electron-volts (eV), which correlates the energy to the electric potential that moves electrons.
Step-by-step explanation:
The energy of the photon emitted when an electron moves from an excited state to a lower energy state can be measured in various units.
The correct option for these units is (b) Electron-volts.
We can deduce this because a photon's energy is directly related to its frequency (E = hf) and can also be described in terms of electron-volts (eV), which is the energy it takes to move an electron through an electric potential of one Volt. According to the references provided, 1 eV is equivalent to 1.602×10-19 J, which allows us to convert energies between these units.
For example, a photon with an energy of 50.0 keV (kilo electron-volts) is equivalent to 50,000 eV.
The photon energy can also be linked to the photoelectric effect, where it's used to calculate the maximum kinetic energy of electrons ejected from a material.