Determine the average rate of decomposition of h3po4 between 10.0 and 40.0 s. express your answer with the appropriate units.

Assume that we have this reaction 2H3PO4⇒ P2O5 + 3H2O
and when the rate of reaction is the speed at which the reactants can be changed and are converted into products So, according to our balanced reaction:

The rate of reaction = rate of formation of P2O5 
and the rate of this reaction also = 0.5 rates of decomposition of H3PO4 
∴ the rate of decomposition of H3PO4 = 2 rates of formation of P2O5
So when the rate of formation P2O5 = 2 x 10^-4 M/s 
So the rate of decomposition of H3PO4 = 2 x 2 x 10^-4
                                                                    = 4 x 10^-4 M/s

Well technically there are two correct answers b and d. but for you its probably b in general.good day sir/ma'am : ) (i know not gender)