Answer:
Option A. Nitrogen-fixing present in the roots of legumes aid enrichment of nitrogen in the soil.
Step-by-step explanation:
Legumes have symbiotic Rhizobium bacteria in their root nodules. The bacteria serve in nitrogen fixation and makes the nitrogen available to plants. When farmer planted legumes and cabbage in the same field that is devoid of fertilizers, nitrogen was made available to the crop by nitrogen fixing bacteria present in root nodules of legumes. This resulted in better growth of cabbage in the field even without any application of fertilizer.