Key Concept: The nitrogen cycle has several named steps, each converting nitrogen between different chemical forms.
Why it's correct: Nitrogen fixation specifically converts atmospheric N$_2$ into ammonia (NH$_3$), making it usable by plants — via Rhizobium bacteria or the industrial Haber-Bosch process.
Why others fail: Nitrification and denitrification act on already-fixed nitrogen, and ammonification releases ammonia from organic matter — none starts from atmospheric N$_2$ gas.