The sequential stage for principal stages of nodule formation are
(1) Rhizobia multiply and colonies the surrounding roots.
(2) Rhizobia attached to epidermal and root hair cells.
(3) Root hair curl and bacteria invades root hair.
(4) Initiation of nodule formation in cortex.
(5) Infection thread is produced carrying bacteria to cortex.
(6) Bacteria released from thread into cells and causes their differentiation in specialized nitrogen fixing cells.
(7) Nodule formed establishes, a direct vascular connection with the host for exchange of nutrients.

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VII → VI → IV → I → III → V → II

I → II → III → V → IV → VI → VII

I → VII → II → VI → III → V → IV

II → III → IV → I → VII → V → VI

answer is D.

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Detailed Solution

Nodule formation involves a sequence of multiple interactions between Rhizobium and roots of the legume plant.

Rhizobia multiply and colonise the surroundings of roots. Rhizobia attached to epidermal and root hair cells. The root-hairs curl and the bacteria invade the root-hair. 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 specialised nitrogen fixing cells. The nodule thus formed, establishes a direct vascular connection with the host for exchange of nutrients.

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