What is the process by which nitrogen is combined with hydrogen to form ammonia?

The process by which nitrogen is combined with hydrogen to form ammonia is known as nitrogen fixation. In this process, atmospheric nitrogen gas (N2) is converted into ammonia (NH3) by certain bacteria, particularly those associated with the roots of leguminous plants, or through industrial means like the Haber-Bosch process.

Nitrification, the choice selected, is a different process that involves the conversion of ammonia into nitrites and then nitrates by specific bacteria in the soil. This is a subsequent step in the nitrogen cycle after ammonia is formed.

Assimilation refers to the process by which living organisms take up nitrogen in the form of nitrates or ammonium and incorporate it into their own biological molecules.

Denitrification involves the conversion of nitrates back into nitrogen gas, thus completing the nitrogen cycle by returning nitrogen to the atmosphere.

Photosynthesis is a completely separate process whereby plants convert carbon dioxide and water into glucose and oxygen using sunlight.

Therefore, the correct understanding of nitrogen fixation highlights its critical role in the nitrogen cycle, where atmospheric nitrogen is ultimately transformed into a form usable by living organisms, specifically ammonia.