I know that the answer is 3P1 but I don't know why

             You are right. The answer is 3Pi.

Reason:

             In this problem all parameters are kept the same. Except of one thing. The only thing which is changing is the number of moles. If you look at the balanced chemical equation 1 mole of  methanol is producing 1 mole of carbon monoxide and 2 moles of  hydrogen gas.

Hence, before reaction we have one mole of gas and after reaction completion we have three moles of product.

According to Ideal gas equation,

P V = n R T

V, R and T are constant hence.

P = n

Hence, three moles of gas will produce 3P.

Let me know if you need more assistance.

10 years is the answer

the answer is b. dilution