Consider the Born-Haber cycle for the formation of an ionic compound given below and identify the compound ( ) formed.
\[
\left[\begin{array}{l}
\mathrm{M}(\mathrm{s}) \stackrel{\Delta \mathrm{H}_{1}}{\longrightarrow} \mathrm{M}(\mathrm{g}) \stackrel{\Delta \mathrm{H}_{2}}{\longrightarrow} \mathrm{M}^{+}(\mathrm{g}) \\
\frac{1}{2} \mathrm{X}_{2}(\mathrm{~g}) \stackrel{\Delta \mathrm{H}_{3}}{\longrightarrow} \mathrm{X}(\mathrm{g}) \stackrel{\Delta \mathrm{H}_{4}}{\longrightarrow} \mathrm{X}^{-}(\mathrm{g})
\end{array}\right] \stackrel{\Delta \mathrm{H}_{5}}{\longrightarrow} \mathrm{Z}
\]

1. A \(\mathrm{M}^{+} \mathrm{X}^{-}\)
2. B \(\mathrm{M}^{+} \mathrm{X}^{-}(\mathrm{s})\)
3. C MX
4. D \(\mathbf{M}^{+} \mathrm{X}^{-}(\mathrm{g})\)