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# Which of the following is free-living aerobic and nonphotosynthetic nitrogen fixing bacterium

$\begin{array}{1 1}(A)\;\text{Rhizobium}\\(B)\;\text{Nostoc}\\(C)\;\text{Azospirillum}\\(D)\;\text{Azotobacter}\end{array}$

Azotobacter
Hence (D) is the correct answer.