The freshwater fern Azolla harbours Anabaena (a blue green alga) in its leaf cavities. Chlorella is simply a green alga. Nostoc is a free living cyanobacteria involved in N₂ fixation and so is Tolypothrix.

Correct Option: D

The freshwater fern Azolla harbours Anabaena (a blue green alga) in its leaf cavities. Chlorella is simply a green alga. Nostoc is a free living cyanobacteria involved in N₂ fixation and so is Tolypothrix.

Nitrogen fixation in root nodules of Alnus is brought about by Frankia. It is a symbiotic nitrogen fixing bacteria in root nodules of several non-legume plants like Casuarina (Australian Pine), Myrica and Alnus (Alder).

Correct Option: C

Nitrogen fixation in root nodules of Alnus is brought about by Frankia. It is a symbiotic nitrogen fixing bacteria in root nodules of several non-legume plants like Casuarina (Australian Pine), Myrica and Alnus (Alder).

Frankia is a symbiotic nitrogen fixer in root nodules of several non-legume plants like Casurina and Alnus.

Correct Option: B

Frankia is a symbiotic nitrogen fixer in root nodules of several non-legume plants like Casurina and Alnus.

Rhodospirillum is a free-living, anaerobic nitrogen fixer bacteria. It can synthesize its food in presence of light under anaerobic conditions. Beijernickia and Azotobacter are free living aerobic nitrogen fixing bacteria. Rhizobium is a symbiotic nitrogen fixing bacteria.

Correct Option: B

Rhodospirillum is a free-living, anaerobic nitrogen fixer bacteria. It can synthesize its food in presence of light under anaerobic conditions. Beijernickia and Azotobacter are free living aerobic nitrogen fixing bacteria. Rhizobium is a symbiotic nitrogen fixing bacteria.

Molybdenum is a micronutrient that plays an important role in nitrogen fixation. It is a part of nitrate reductase enzyme that helps in nitrogen fixation.

Correct Option: A

Molybdenum is a micronutrient that plays an important role in nitrogen fixation. It is a part of nitrate reductase enzyme that helps in nitrogen fixation.