Nitrogen fixation in root nodules of Alnus is brought about by which of the following organism ?

Frankia

The correct answer is option (1) -Frankia

Frankia is a genus of nitrogen-fixing bacteria that live in symbiosis with actinorhizal plants, similar to the Rhizobium bacteria found in the root nodules of legumes in the family Fabaceae. Frankia also initiate the forming of root nodules. Symbiotic nitrogen fixation is accomplished by Rhizobium species, which occurs on the roots of leguminous plants. Certain non-leguminous plants like Alnus also form nodules to fix nitrogen. The best known example in temperature involved in nodule formation is an actinomycetes, the Frankia.