d
In one \(\mathrm{Mg}_{3}\left(\mathrm{PO}_{4}\right)_{2}\) molecule no of oxygen atoms \(=8\)

So if there is \(M\; mole\) of \(Mg_3\left(\mathrm{PO}_{4}\right)_{2}\) molecules then no of moles of oxygen atoms is \(=\mathrm{M} \times 8\)

According to question,

\(M \times 8=0.25\)

\(\Rightarrow M=0.03125=3.125 \times 10^{-2}\)