What is free nitrogen?
a. nitrogen that is ready for plants to use
b. nitrogen that has been fixed by bacteria
c. nitrogen that cannot be used by organisms
d. nitrogen that is returned to the soil by decomposers

Here, we are required to identify what free nitrogen is.

Molecular nitrogen (N2) in its molecular form, consisting of two nitrogen atoms bound together with a tripple bond as a result of its 5 electrons valency is simply reffered to as Free nitrogen.

Due to its stability, N2 is quite unreactive, and takes a lot of energy (high bond dissociation energy) to break atoms which form the Nitrogen molecules apart.

A number of plants, including beans and other legumes have growths (knobby) on their roots called rhizomes that are capable of “fixing” nitrogen, or break up the nitrogen molecules so that the reactive N atoms to react with organic chemicals to form nitrogen containing organic compounds. Among these are the amino acids necessary for life to begin and which are the building blocks from which DNA is made.

Consequently, nitrogen is only referred to as fixed if it exists in an organic compound, otherwise it is considered to be free nitrogen(N2).

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the answer to your question is b

the correct answer is d