Answer:
The correct answer would be "to convert free nitrogen into usable nitrogen".
Symbiotic bacteria such as Rhizobium live in the roots of the plants like soybean and peas help in the process of biological nitrogen fixation.
It is the process by which bacteria convert the atmospheric nitrogen to the form which can be used easily by the plants. In return, they get energy-producing nutrients such as glucose.
The reaction is usually catalyzed by the enzyme nitrogenase.
The chemical reaction can be written as:
N₂ + 16ATP + 8e⁻ + 8H⁺ → 2NH₃ + H₂ + 16ADP + 16Pi