ANS:A - oxidation of ammonia.

Nitric acid is produced on a commercial scale in a fertiliser plant through the oxidation of ammonia. This process, known as the Ostwald process, involves the following steps:

1. Ammonia Oxidation: Ammonia (NH3) is oxidized to nitric oxide (NO) using a platinum catalyst at high temperatures. The reaction is typically carried out at temperatures around 850-1000°C: 4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(g)
2. Nitric Oxide Oxidation: Nitric oxide (NO) is further oxidized to nitrogen dioxide (NO2) by oxygen (O2) from the air in the presence of excess air. This reaction occurs spontaneously: 2NO(g) + O2(g) → 2NO2(g)
3. Absorption of Nitrogen Dioxide: The nitrogen dioxide (NO2) produced in the previous step is absorbed in water to form nitric acid (HNO3): 3NO2(g) + H2O(l) → 2HNO3(aq) + NO(g)