The Bohr model of the atom proposed the existence of

The question is asking about the Bohr model of the atom and what it proposed. The Bohr model of the atom was proposed by Niels Bohr in 1913. It is a model that describes the structure of the atom, with the nucleus at the center and electrons orbiting the nucleus in specific energy levels or shells. Therefore, the answer to the question is "electron shells". The Bohr model proposed the existence of electron shells, which are specific energy levels that electrons occupy around the nucleus of an atom. These shells are quantized, meaning that they can only have certain energy values, and electrons can move between them by absorbing or releasing energy. Option A, "the nucleus", is not specific to the Bohr model and was already known to exist before the development of the Bohr model. Option C, "nucleons", refers to the collective term for the particles in the nucleus (protons and neutrons), which are not part of the Bohr model's description of the electron shells. Option D, "neutrons", is a type of nucleon, but again, it is not part of the Bohr model's description of the electron shells.