As this reaction takes place at higher temperatures, it is observed that the equilibrium shifts toward the products. the reaction is

The answer is: endothermic reaction.

This reaction is endothermic (enthalpy is higher than zero), which means that heat is added.  

According to Le Chatelier's principle when the reaction is endothermic heat is included as a reactant and when the temperature increased, the heat of the system increase, so the system consume some of that heat by shifting the equilibrium to the right, producing more products.

the answer is b. dilution