When $\mathrm{NaOH}$ is added to solution(C) calcium nitrate, double displacement reaction takes place, a white ppt. of calcium hydroxide is formed which is insoluble in excess of ${\mathrm{NH}}_4\mathrm{OH}$ solution.
$2\mathrm{NaOH}\left(\mathit{a}\mathit{q}\right)+\mathrm{Ca}{\left({\mathrm{NO}}_3\right)}_2\left(\mathit{a}\mathit{q}\right)\to$ $\underset{\mathrm{White}\mathrm{ppt}}{\mathrm{Ca}{\left(\mathrm{OH}\right)}_2}+2{\mathrm{NaNO}}_3\left(\mathit{a}\mathit{q}\right)$
The cation is ${\mathrm{Ca}}^{2+}$.