Final answer:
Niels Bohr proposed that an electron in an excited state of an atom would emit a single photon when it transitions back to a lower energy level, which is a behavior referred to as single photon emission.
Step-by-step explanation:
According to Niels Bohr's model of the atom, he proposed that an electron in an excited state would emit a single photon when it transitions back to a lower energy level. Ordinarily, an atom is in the ground state, which is the state of lowest possible energy with the electron in the innermost orbit. When an atom absorbs energy, the electron moves to a higher energy level, referred to as excitation. Eventually, the electron will return to the ground state; during this transition, it emits a photon corresponding to the energy difference between the excited state and the ground state.
This behavior results in the emission of light and is observed in phenomena such as the Balmer series of hydrogen, which are emission or absorption lines visible in the hydrogen spectrum. Therefore, Niels Bohr proposed that the behavior of an electron in an excited state pertains to single photon emission.