Which one of the following produces nitrogen fixing nodules on the...

The most common association on roots is nodules. These nodules are small outgrowths on the roots. Frankia is a genus of nitrogen-fixing, free-living, filamentous bacteria. It produces nitrogen-fixing nodules on the roots of non-leguminous plants (e.g., Alnus).

The species of rod-shaped Rhizobium forms nodules with the roots of several legumes and helps in nitrogen fixation.