I need help understanding how to differentiate between isomers and resonance structures in skeletal structures.

Question

Specifically, here is an image to a problem with the answers:

i.imgur. com /mz3URSa.jpg
(remove spaces)

The left most structure is the reference structure and the others are the ones I am supposed to identify as resonance, isomer, or neither, relative to the reference.

I originally thought that C was the resonance structure and that A and B were isomers because the formal charge had relocated, but that's not the case. Why is this so?

Thanks in advance.