Which of the following is not a nitrogen fixing microorganism?

The correct answer is Option (3) → Acetobacter

Option (1) Azotobacter — A nitrogen-fixing microorganism. A free-living soil bacterium.

Option (2) Oscillatoria — A nitrogen-fixing microorganism. A cyanobacterium.

Option (3) Acetobacter — Not a nitrogen-fixing microorganism. Acetobacter aceti is used to make acetic acid (vinegar).

Option (4) Nostoc — A nitrogen-fixing microorganism. A cyanobacterium, important in paddy fields.

NCERT Extract:
Microbes as Biofertilisers – “… Cyanobacteria are autotrophic microbes widely distributed in aquatic and terrestrial environments many of which can fix atmospheric nitrogen, e.g. Anabaena, Nostoc, Oscillatoria, etc. In paddy fields, cyanobacteria serve as an important biofertiliser. Blue green algae also add organic matter to the soil and increase its fertility.”
Microbes as Biofertilisers – “… Other bacteria can fix atmospheric nitrogen while free-living in the soil (examples Azospirillum and Azotobacter), thus enriching the nitrogen content of the soil.”
Chemicals, Enzymes and Other Bioactive Molecules – “… Examples of acid producers are Aspergillus niger (a fungus) of citric acid, Acetobacter aceti (a bacterium) of acetic acid; Clostridium butylicum (a bacterium) of butyric acid and Lactobacillus (a bacterium) of lactic acid.”