Resonance structures differ by resonance structures differ by number of atoms only number of electrons only number and placement of electrons placement of electrons only placement of atoms only

The answer to this question would be: placement of electrons only

An atom with resonance structure or mesomerism has one or more different structure with different placement of the electron. The atom should have a same amount of electron but only differ in the electron position. This will result in a different Lewis structure model. In this structure, you can find π electrons pair.

Wow that a lot of text for my tiny so i no wanna answer