Below are the electronegativties and the differences between the given values in order to arrive to the final answer.
(1) phosphorus: 2.19 lithium: 0.98 ; difference = 1.21
(2) chlorine: 3.16 cesium: 1.12 ; difference = 2.04
(3) aluminum: 1.61 oxygen: 3.44 ; difference = 1.83
(4) bromine: 2.96 bromine: 2.96 ; difference = 0
(5) oxygen: 3.44 strontium: 0.95 ; difference = 2.49
Therefore, the order of electronegativity difference from biggest to least is 5, 2, 3, 1, and 4