i’ve never seen it done like this, but i can say that the answer is most likely B.
the 3 in Na3PO4(s) belongs to the sodium atom (Na). so in any of these equations the 3 would have to be with Na.
A- the 3 is along w the PO4, which would make it part of that bond
C- there is no 3 at all for Na in this choice, making it incorrect
D- Again, the 3 is placed on the other half of the bond
*THE REASON THERE IS A 3- IS BECAUSE THAT IS THE CHARGE OF PO4, IT IS DIFFERENT THAN THE 3 FOR NA*
