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maulmarDanight
Answered

Model in which electrons are found in fixed levels around the nucleus and require energy to move from level to level:

ABohr model
CQuantum mechanical model
BRutherford model
DFree-electron model

Verified Answer :

Tiendi

this is Bohr's model of electron shells - this progressed later into the quantum mechanical model which disproved 'shells' but continued to refer to the energy levels as 'shells' for this historic reason 

= a. Bohrs model