Final answer:
When a hydrogen atom's electron moves from a higher-energy orbit to a lower-energy one in Bohr's model, it emits a photon of light.
Step-by-step explanation:
According to Niels Bohr's model of the hydrogen atom, the event that occurs when the atom's electron drops from a higher-energy level to a lower-energy level is the emission of a photon (light). This is because electrons in a hydrogen atom orbit the nucleus in specific energy levels or shells. When an electron transitions from a higher-energy orbit to a lower-energy one, it must release energy. The energy difference between the two levels is emitted as a photon of light, which has a frequency corresponding to this energy change.
The answer to the question is: b. A photon (light) will be emitted.