Nitrogen Fixation

Let's analyze the organisms: A. Azotobacter is a nitrogen-fixing bacterium, so it can fix nitrogen.
B. Oscillatoria is a type of cyanobacteria (blue-green algae) and can fix nitrogen.
C. Anabaena is also a nitrogen-fixing cyanobacterium.
D. Volvox is a green alga that cannot fix nitrogen. It relies on the environment for nitrogen and does not have the ability to fix it.
E. Nostoc is a nitrogen-fixing cyanobacterium. Thus, the correct answer is D only.