Final answer:
An electron moving from the 2nd to the 4th energy level in a hydrogen atom absorbs a photon, based on the principles of the Bohr model.
Step-by-step explanation:
When an electron in a Bohr model hydrogen atom jumps from the 2nd energy level to the 4th level, the process involves the absorption of a photon. According to the Bohr model, electrons reside in discrete energy levels, and a change in energy level requires the transfer of a specific amount of energy. In this case, the electron is moving to a higher energy level, which means that it must absorb energy in the form of a photon. This is contrary to the process of emission, which occurs when an electron drops to a lower energy level, releasing a photon with energy corresponding to the difference of the two levels.