Option B: Bohr's atomic theory.
According to Bohr’s atomic model, electrons orbit the nucleus and they exist in constant energy state called stationary states. As they do not radiate energy, they orbit at constant distance from the nucleus. Bohr examine the emission spectrum of hydrogen, according to his observations he explained that on adding energy, electrons can move in different orbits and after removing the energy they came back to their original state by emitting the equal amount of energy.
Thus, in an atom, electrons move around nucleus in fixed paths with fixed energy called orbits . When it is in its original state, the energy is very low, and the state is known as ground state (closer to the nucleus). After gaining energy, electron gets excited to higher energy states known as excited state.
Since, it explains about the chemistry of atom, it is the basic model for chemistry.
