HI!
The correct order would be:
(least electronegative) Cesium -> Strontium -> Copper -> Lead -> Bromine -> Krypton (most electronegative)
Electronegativity is that chemical property of an atom that describes its tendency/ability to attract an electron to itself. The higher the electronegativity of the atom, the more its tendency to attract an ion to itself, increasing the likelihood of forming a negative ion in a compound. The difference in the electronegativity values of two atoms forming a compound can help indicate the type of bond that will form between them: covalent or ionic. If the difference is greater than 2, the bond found to be ionic.
The electronegativity values of the atoms in the question are as follows:
Cs Cesium 0.79
Sr Strontium 0.95
Cu Copper 1.90
Pb Lead 2.33
Br Bromine 2.96
Kr Krypton 3.00
Hope this helps!