The reaction A(g) + 2B(g) ---> C(g) is an elementary reaction. In an experiment involving this reaction, the initial partial pressures of A & B are Pa = 0.4 atm & Pb = 1 atm. When pressure of C becomes 0.3 atm in the reaction the rate of reaction relative to the initial rate is :-

(A) 1/12 (B) 1/50 (C) 1/25 (D) None of these

Please recheck this question, some information is missing in this question.. As the reaction

 A(g) + 2B(g) ---> C(g)

is an elementary reaction, therefore we can deduce the rate law as

 -d[A] / dt = k [A][B]²  (1)

where k is the order of reaction. As P_A = 0.4 atm & P_B = 1 atm, then on substitution of these values in the above equation we get

 -d[A] / dt = 0.4k

When we write the rate law in terms of concentration of C, then it will be written as 

 d[C] / dt = k [A][B]²  (2)

here we need to know the concentration of A and B in order to calculate the ration of equation (1) and (2) respectively. If not that, we need to know in how much time has the concentration of C reached 0.3 atm, and whether C was already present in the system or not.