Which of following organisms cannot fix nitrogen?

A. Azotobacter

B. Oscillatoria

C. Anabaena

D. Volvox

E. Nostoc

Choose the correct answer from the options given below:

The correct option is D only.

To understand why Volvox is the correct answer, let us look at the ability to fix nitrogen among different organisms:
1. Nitrogen Fixation is a Prokaryotic Process: Biological nitrogen fixation is the process of converting atmospheric nitrogen into ammonia. This process is catalyzed by the nitrogenase enzyme complex, which is strictly found in certain prokaryotic organisms (bacteria and cyanobacteria). Eukaryotes do not possess the genes required to synthesize nitrogenase and therefore cannot fix nitrogen on their own.
2. Analyzing the Options:
- Azotobacter (A): This is a genus of free-living, aerobic, nitrogen-fixing bacteria found in soil.
- Oscillatoria (B): This is a genus of filamentous cyanobacteria (blue-green algae) capable of nitrogen fixation, typically under microaerophilic or anaerobic conditions.
- Anabaena (C): This is a genus of filamentous cyanobacteria that forms specialized, thick-walled cells called heterocysts to fix atmospheric nitrogen, often living in symbiotic relationships with plants like Azolla.
- Volvox (D): This is a genus of multicellular eukaryotic green algae. Because it is a eukaryote, it lacks the nitrogenase enzyme and cannot fix nitrogen.
- Nostoc (E): This is a genus of cyanobacteria that forms gelatinous colonies and fixes nitrogen using specialized cells called heterocysts.

Since Volvox is the only eukaryote among the listed organisms, it is the only one that cannot fix nitrogen. Thus, "D only" is the correct choice.