A. when an electron in an atom is excited to a higher energy state
Explanation;
Lines on the atomic spectrum are based on the electron transitions between energy levels, such that when an electron drops an energy level a photon is released resulting in an emission line and when an electron absorbs a photon and rises an energy level an absorption line is observed on the spectrum.
Therefore; an absorption spectrum is a spectrum of absorption lines or bands which occurs when electrons move to a higher orbit by absorbing energy. Each atom has a unique spacing of orbits and can emit or absorb only certain energies or wavelengths.
