Explanation:

Groundnut ($Arachis$ $hypogaea$) is a legume crop. Legumes form a symbiotic relationship with Rhizobium bacteria, which reside in root nodules and are capable of fixing atmospheric nitrogen ($N_2$) into a form usable by plants (ammonia, $NH_3$). This biological nitrogen fixation significantly reduces the need for synthetic nitrogen fertilizers. Therefore, inoculating groundnut seeds or soil with Rhizobium culture is a priority to enhance nitrogen fixation and improve crop yield. Options A (Azospirillum) and D (Phosphobacteria) are beneficial microbes but are not the primary symbiotic nitrogen fixers for legumes. Option C (Mycorrhiza) forms symbiotic relationships with plant roots to enhance nutrient (especially phosphorus) uptake, but it is not directly involved in nitrogen fixation for groundnut.