The enantiomers can be separated because 1,2-dimethylazetidine exhibits resonance, making the nitrogen sp² hybridized, and preventing the interconversion of the enantiomers.

The enantiomers can be separated because the nitrogen in the ring cannot meet the 120° bond angle requirement for amine inversion, preventing the interconversion of the enantiomers.

The enantiomers can be separated because the nitrogen needs to be protonated first as a requirement for amine inversion, preventing the interconversion of the enantiomers.

The enantiomers can be separated because nitrogen is an asymmetric center. The lone pair does not affect the interconversion of the enantiomers.