Final answer:
Niels Bohr proposed the model of the atom with shells, known as the Bohr model.
Step-by-step explanation:
The correct answer is A. Bohr. Niels Bohr proposed a theoretical model for the hydrogen atom known as the Bohr model, which explained the emission spectrum of hydrogen. According to Bohr's model, electrons move in circular orbits around the nucleus and occupy specific energy levels or shells. This model was based on Rutherford's nuclear atom theory, which demonstrated the presence of the atomic nucleus.