The compound in which the cation is isoelectronic with the anion is

Which of the following substances is the most ionic?

Two elements $X$ and $Y$ have the following electron configurations:
$X = 1s^2, 2s^2 2p^6, 3s^2 3p^6, 4s^2$
$Y = 1s^2, 2s^2 2p^6, 3s^2 3p^5$
The compound formed by the combination of $X$ and $Y$ is:

Which of the following substances is ionic in nature?

Two elements have electronegativity of $1.2$ and $3.0$. The bond formed between them would be:

An ionic compound $A^{+} B^{-}$ is most likely to be formed when