Explanation

(d)
Phosphorous is estimated as $Mg_{2}P_{2}O_{7}$
$P\ \ \underset{\mathrm{\Delta}}{\overset{{HNO}_{3}}{\rightarrow}}\ H_{3}PO_{4}$
$H_{3}PO_{4} + Mg^{2 +} + NH_{4}OH \rightarrow MgNH_{4}PO_{4}$
$2MgNH_{4}PO_{4} \rightarrow Mg_{2}P_{2}O_{7} + H_{2}O + 2NH_{3}$
% of P $= \frac{62 \times wt.of\ Mg_{2}P_{2}O_{7} \times 100}{222 \times w}$