A filamentous nitrogen fixing bacteria is present in root nodules of flowering plants is
A. Cicer arietinum
B. Casuarina equisetifolia
C. Cycas revoluta
D. Crotalaria juncea

Hint:The nitrogen-fixing bacteria assists in the conversion of the atmospheric nitrogen to ammonium compounds and provides it to the plant. This process is called nitrogen fixation. There are a number of organisms involved in the process through which it is done.

Complete answer:
There are two types of nitrogen-fixing bacteria (1) free-living nitrogen-fixing bacteria and (2) symbiotic nitrogen-fixing bacteria. Symbiotic bacteria are present in the root nodules of the leguminous plants. They provide nitrogen to the plants and gain nutrition in exchange for the plant.
Casuarina equisetifolia is an evergreen pine tree belonging to the casuarina genus and the species is equisetifolia. Filamentous symbiotic nitrogen-fixing bacteria is found in the root nodules of the tree Casuarina equisetifolia. It is therefore a symbiotic mode of nitrogen fixation.

The correct answer is option b) Casuarina equisetifolia.

Additional information:
The bacteria in the root nodules of Cicer arietinum is rhizobium and it is nonfilamentous.
The bacteria that live in the root nodules of Cycas revolute is Mesorhizobium and it is nonfilamentous. The bacteria present in the root nodules of Crotalaria juncea
is Bradyrhizobium and Methylobacterium which is non-filamentous nitrogen-fixing bacteria too.

Note : As the name suggests nitrogen-fixing bacteria helps in the fixation of nitrogen and provides it to the plants. The nitrogen present in the atmosphere is unavailable to the plants until it is fixed by these bacterial forms. It is after the fixation process that the plants can utilize the nitrogen.