The pair of elements which on combination is most likely to form an ionic compound is :

Electrostatic attraction between a metal cation and a non-metal anion of similar size results in the formation of ionic compounds. They make a more covalent bond if the cation is little and the anion is very large due to the size difference.

K is a metal, O is a non-metal, and they both exist as ${\mathrm{K}}^{+}$ and ${\mathrm{O}}^2$. consequently have an ionic bond.