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. $\;2\,\mathrm{NaOH}(\mathrm{aq}) + \mathrm{Ca}(\mathrm{NO}_3)_2(\mathrm{aq}) \rightarrow$ $\;\underset{\text{White ppt}}{\mathrm{Ca}(\mathrm{OH})_2} + 2\,\mathrm{NaNO}_3(\mathrm{aq})$ The cation is $\mathrm{Ca}^{2+}$.