How many moles of magnesium phosphate $Mg(PO_4)_2$ will contain 0.25 mole of oxygen atom ?

Question

$\begin{array}{1 1}(a)\;2.5\times 10^{-2}&(b)\;0.02\\(c)\;3.125\times 10^{-2}&(d)1.25\times 10^{-2}\end{array}$