ANSWER
D. Emission Spectrum
EXPLANATION
When an electron in an atom falls from a higher level to a lower level, energy is decreased and a photon is emitted. Photons are the carriers of electromagnetic and are the form in which electromagnetic radiation is transmitted/emitted.
When an electron falls to a lower level in an atom, light from a wavelength belonging to the emission spectrum is emitted.
Therefore, the answer is option D.
