Three resonating structures for buta-1,3-diene is given:

The decreasing energy order is:

Higher stability $\Rightarrow$ Lower energy

I $\Rightarrow$ uncharged structure, most stable, less energy

II $\Rightarrow$ uncharged structure, most stable, less energy
II is more stable than III, because unlike charges are more closer
to each other or lesser charge separation means more stable.
Therefore energy of II < III.
Thus order is:
III>II>I