26) E
Since temperature is proportional to pressure, pressure will initially increase to allow both gases to react.
Now let us write out the reaction equation
C₂H₄ + H₂ -> C₂H₆
Using the equilibrium rules, we can see that moles of products is ½ of moles of reactants. This means that as the reaction progresses, pressure will decrease.
Hence, E is the answer.