Which defines the energy levels in the quantum mechanical model?

The energy levels in the quantum mechanical model refer to the quantized states in which electrons can exist around an atom's nucleus. Each energy level corresponds to a specific amount of energy that an electron can have and is associated with particular orbital shapes and sizes. Therefore, option C is correct as it articulates the direct relationship between energy levels and the sizes of electron orbitals.

As energy levels increase, so do the orbital sizes, which means that electrons occupy higher energy states at further distances from the nucleus. This relationship between energy levels and orbital sizes is a crucial aspect of quantum mechanics and helps explain the distribution and behavior of electrons in atoms.