On the basis of electronegativity, which of these pairs of elements is most likely to form polyatomic ions?
possible answers:
cs and cl
na and br
o and p
ba and o

Correct  O and P

Reason: 
The nature of bond depends on the electronegativity difference of constituent atoms. If difference in electronegativity of constituent atoms is more, it will result in ionic bonding. Alternatively, if difference in electronegativity of constituent atoms is less, it will prefer covalent bonding. Further,  covalently bonded elements are most likely to form polyatomic ions.

In present case, electronegativity difference of O and P is minimum. Hence, they are most likely to form polyatomic ions.