Answer
$CaO$
Work Step by Step
In NaCl, one electron is exchanged, and in CaO, two ($Ca^{2+}, O^{2-}$) are exchanged.
From Couloumb's Law: $F=-k\frac{q_1q_2}{d^2}$
Since $q_1$ and $q_2$ are greater for Calcium and Oxygen than for Sodium and Chlorine, the attractive force is greater for CaO.
