In this case, when we want to balance chemical reactions such as in this case, the idea is to equal to number of atoms of each element at each side of the equation according to the lay of conservation of mass, just as shown below:
Because we have four phosphorous and ten oxygen atoms at each side.
D. this is the correct answer because the question is showing a comparison of sizes rather than the actual sizes which eliminates B. A and C have no relevance.
