Mature root nodules are made largely of

✅ Explanation: Mature root nodules, the structures formed on the roots of legumes in symbiotic association with nitrogen-fixing bacteria (rhizobia), are primarily composed of tetraploid cells. These cells are formed due to the repeated division of the infected plant cells without subsequent cytokinesis (cell division). The increased ploidy level is believed to enhance the metabolic activity of these cells, supporting the energy-intensive process of nitrogen fixation.