0.75 mole of solid $A_4$ and 2 mole of gaseous $O_2$ are heated to react completely in sealed vessel to produce only one gaseous compound B.After the compound is formed the vessel is brought to initial temperature,the pressure is found to half the initial pressure.What is molecular formula of compound?

Question

$\begin{array}{1 1}(a)\;A_3O_4&(b)\;A_2O_6\\(c)\;AO_2&(d)\;A_2O_4\end{array}$