Concept:The Bohr model describes electrons moving in fixed circular paths around the nucleus, called energy levels or shells.
Explanation:Bohr proposed that electrons revolve around the positively charged nucleus in definite, allowed orbits.
Each orbit corresponds to a fixed energy level, known as an electron shell.
These shells are labelled as K, L, M, N, etc., starting from the nucleus outward.
Electrons can jump between shells by absorbing or releasing energy.
This idea of discrete shells was a major step beyond earlier atomic models.
The existence of the nucleus was already proposed by Rutherford.
Nucleons and neutrons were discovered later through other experiments.
Therefore, the new proposal in Bohr’s model is the existence of electron shells.
Answer:B. electron shells