Question

An anaerobic nitrogen fixing bacterium is
A. Azotobacter
B. Chlorobium
C. Rhodospirillum
D. Clostridium

Answer 1

Hint: Nitrogen-fixing bacteria, microorganisms capable of reworking atmospheric nitrogen into fixed nitrogen (inorganic compounds usable by plants).

Complete answer:
Nitrogen fixation is a process by which molecular nitrogen within the air is converted into ammonia or related nitrogenous compounds in soil. Atmospheric nitrogen is molecular dinitrogen, a comparatively nonreactive molecule that's metabolically useless to all or any but some microorganisms. Biological organic process converts N$_2$ into ammonia, which is metabolized by most organisms.
Nitrogen Fixers: Some prokaryotes like bacteria and cyanobacteria can fix atmospheric nitrogen. They're called nitrogen fixers or diazotrophs. Diazotrophs could also be asymbiotic (free living) or symbiotic:
Nitrogen Fixing Bacteria: Azotobacter, Beijerinckia and Clostridium (anaerobic) are saprophytic bacteria that perform organic processes. Desulfovibrio is chemotrophic nitrogen fixing bacteria. Rhodopseudomonas, Rhodospirillum and Chromatium are nitrogen fixing photoautotrophic bacteria. These bacteria add up to 10-25 kg of nitrogen/ha/annum.

Nitrogen Fixing Cyanobacteria: Many cyanobacteria perform biological processes, e.g., Anabaena, Nustoc, Aulosira, Cylindrospermum, Trichodesmium.

Symbiotic Nitrogen Fixing Cyanobacteria: Anabaena and Nostoc species are common symbionts in lichens, Anthoceros, Azolla and cycad roots. Azolla pinnata (a water fern) has Anabaena azollae in its fronds. It's often inoculated to rice fields for organic process.

Symbiotic Nitrogen Fixing Bacteria: Rhizobium is aerobic, gram negative nitrogen fixing bacterial symbionts of Papilionaceous roots. Sesbania rostrata has rhizobium in root nodules and Aero Rhizobium in stem nodules. Frankia is symbiont in root nodules of the many non-leguminous plants like Casuarina and Alnus.

Symbiotic Nitrogen Fixation: Both Rhizobium sp. and Frankia fix atmospheric di-nitrogen.

Diazotroph: Diazotrophs are widespread within domain Bacteria including cyanobacteria, also as green thiobacillus, Azotobacteraceae, rhizobia and Frankia. Several obligately anaerobic bacteria fix nitrogen like Clostridium spp. Some archaea also fix nitrogen.

Therefore, the correct option is d, Clostridium.

Note: Cyanobacteria inhabit nearly all illuminated environments on Earth and play key roles within the carbon and biological process of the biosphere. Cyanobacteria use various inorganic and organic sources of combined nitrogen, like nitrate, nitrite, ammonium, urea, or some amino acids. Several cyanobacteria strains are capable of diazotrophic growth. Biological processes by cyanobacteria in coral reefs can fix twice the maximum amount of nitrogen as on land—around 660 kg/ha/year. The colonial marine cyanobacterium Trichodesmium is believed to fix nitrogen on such a scale that it accounts for half the biological process in marine systems globally.