The given diagram shows the development of root nodule in soyabean. Thus structures are marked as A, B, C and D.

The root nodules formation in legumes involve following events:
(i) Rhizobia multiply and colonize the surroundings of roots and get attached to epidermal and root hair cells.
(ii) The root-hairs curl and the bacteria invade the root-hair.
(iii) An infection thread is produced carrying the bacteria into the cortex of the root, where they initiate the nodule formation in the cortex of the root. Then the bacteria are released from the thread into the cells which leads to the differentiation of specialized nitrogen fixing cells.
(iv) The nodule thus formed, establishes a direct vascular connection with the host for exchange of nutrients.