chemical equilibrium and exothermic reaction
Since, H2 is added at the equilibrium state, it's initial pressure will be more and starts decreasing as it goes into the reaction with N2. This is clearly depicted by graph A. While the pressure of N2 starts decreasing from the level it was in equilibrium state, represented by graph C. Also, the reaction shifts forwards in order to gain equilibrium, pressure of NH3 will increase. So, the correct answer would be,


  A = H2, B = NH3, C = N2