The role of 'Molybdenum' in Haber's synthesis is

Catalysts are substances that increase the rate of a reaction.

Promoters are substances that enhance the activity of a catalyst while inhibitors poisons the catalyst and decrease the activity of the catalyst.

In Haber’s process, a catalyst is used to increase the rate of the reaction to get ammonia as the product at a much faster rate, finely divided iron is used. To enhance the activity of iron, the promoter used is molybdenum.

Therefore, option D is the correct answer.