What does the term ‘nitrogen fixation’ mean ?

The correct option is: Conversion of atmospheric nitrogen to a more usable form for living organisms.

Explanation:

Nitrogen is an essential element for life, as it is a crucial component of proteins, nucleic acids (DNA and RNA), and other cellular structures. Although atmospheric nitrogen gas (${\text{N}}_2$) makes up about 78% of Earth's atmosphere, most living organisms (such as plants and animals) cannot utilize nitrogen directly in its gaseous form because of the strong triple chemical bond holding the two nitrogen atoms together.

Nitrogen Fixation is the biological or chemical process by which inert atmospheric nitrogen gas is converted into reactive and usable forms, such as ammonia (${\text{NH}}_3$), ammonium (${\text{NH}}_4^{+}$), or nitrates (${\text{NO}}_3^{-}$). These fixed forms can then be easily absorbed by plants from the soil and incorporated into biological macromolecules, entering the food chain for other living organisms.

This process is predominantly carried out naturally by nitrogen-fixing bacteria (such as Rhizobium in root nodules of legumes or free-living Azotobacter), cyanobacteria, and to a lesser extent by physical phenomena like lightning strikes.