4. Presence of oxygen increases nitrogen fixation in Rhizobium.

The correct option is B False

1. Rhizobium is a nitrogen-fixing bacteria that lives in the roots of leguminous plants.
2. It fixes atmospheric nitrogen and gives it to the plant and in return it gets food and shelter from the plant.
3. Thus, it serves a symbiotic relationship with the plant. But this nitrogen fixation is not dependent upon the presence of oxygen in the air.

So the above statement is false.