Three resonating structures for buta-1,3-diene is given:The decreasing energy order is:

Higher stability ⇒ Lower energyI ⇒ uncharged structure, most stable, less energyII ⇒ uncharged structure, most stable, less energyII is more stable than III, because unlike charges are more closerto each other or lesser charge separation means more stable.Therefore energy of II < III.Thus order is:III>II>I