What type of bacteria is $Rhizobium$?

Symbiotic nitrogen-fixing bacterium is . . . . . . .

Which of the following is a symbiotic nitrogen fixer?

Arrange the stages of root nodule formation in the correct sequence:
$I-$ Infection thread is produced.
$II-$ Bacteria are carried to the cortex of the root.
$III-$ Bacteria attach to the root hair.
$IV-$ Bacteria are released from the thread into the cells.
$V-$ Differentiation into nitrogen-fixing cells occurs.

The nitrogen-fixing symbiotic organism present in $Azolla$ is:

Which bacteria are present in the structure shown below?