Answer: A. when an electron in an atom is excited to a higher energy state
Explanation:
An absorption spectrum consists of dark lines on a visible background. The dark lines indicate the wavelength which is absorbed by the electron to jump from lower energy state to higher energy state.
Electron requires energy to jump to higher state. Specific wavelength is absorbed by the electron which appears as dark line. This wavelength is equivalent to energy difference between the two levels the electron is jumping.