How many moles of magnesium phosphate, $Mg_3 (PO_4)_2$ will contain 0.25 mole of oxygen atoms ?

Question

( A ) $2.5 \times 10^{-2}$
( B ) $3.125 \times 10^{-2}$
( C ) $1.25 \times 10^{-2}$
( D ) $0.02$