The correct option is A Nitrogen
Rhizobium are soil bacteria, that fix nitrogen (diazotrophs) after becoming established inside root nodules of legumes (Fabaceae). In order to express genes for nitrogen fixation, Rhizobium require a plant host; they cannot independently fix nitrogen. In general, they are Gram-negative, motile, non-sporulating rods. In presence of nitrogen they can fix nitrogen. So, supplying nitrogen to them can be helpful for nitrogen fixation.