The correct option is: (d) N2 fixation.
Explanation:
Rhizobium is the nitrogen fixing bacteria present in root nodules of legumes. Roots of a legume secrete chemical attractants flavonoids and betaines. Bacteria collect over the root hairs. release nod factors that causes curling of root hairs around the bacteria, degradations of cell wall and formation of an infection thread enclosing bacteria. The infected cortical cells differentiate and start dividing. It produces swelling of nodules and the infected cells enlarge. Bacteria stop dividing and form irregular polyhedral structures called bacteroids. The host cells develops a pinkish coloured pigment called leg haemoglobin. It is an oxygen scavenger and protects nitrogenase from oxygen. Symbiotic nitrogen fixation requires co-operations of 'Nod' genes of legumes, 'nod' 'nif' and 'fix' genes of bacteria.
