The single largest industrial application of dihydrogen is:

Dihydrogen ($$\text{H}_2$$) has several industrial applications including the synthesis of ammonia, production of metal hydrides, as rocket fuel, and in the synthesis of various chemicals. However, the single largest industrial application needs to be identified.

The Haber-Bosch process for the synthesis of ammonia consumes the largest quantity of dihydrogen industrially. In this process, nitrogen and hydrogen react at high temperature and pressure in the presence of an iron catalyst: $$\text{N}_2 + 3\text{H}_2 \xrightarrow{\text{Fe catalyst}} 2\text{NH}_3$$.

Ammonia production is one of the most important industrial chemical processes in the world, as ammonia is the starting material for fertilisers, which feed a large fraction of the global population. The scale of ammonia production far exceeds any other use of dihydrogen, making it the single largest industrial application.

The correct answer is option (3): in the synthesis of ammonia.