What is the best description of the change that occurs when ${\mathrm{Na}}_2\mathrm{O}\left(\mathrm{s}\right)$ is dissolved in water?

Oxide ion donates a pair of electrons

${\mathrm{Na}}_2\mathrm{O}+{\mathrm{H}}_2\mathrm{O}⟶2\mathrm{NaOH}$

Oxide number $\begin{array}{l}+1-2\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}-2\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}+1+2\end{array}$of $\mathrm{Na}$ and oxygen

No change in oxidation number, so (a) and (b): are not true

(b) is also not true.

Oxide ion donates a pair of electrons thus changes to ${\mathrm{OH}}^{-}$