Answer the question on basis of passage given below :
Vitamins represent a group of organic compounds which are needed in small amount for the healthy growth and maintenance of our body. These are not synthesised by the body and are supplied in the diet. The deficiency of vitamins can be supplemented with medicines.
The pernicious anaemia disease is associated with the deficiency of :

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.