Assertion : Amount of cyanocobalamine required daily is 3 mcg
Reason : Its deficiency causes pernicious anaemia

The correct option is B Both assertion and reason are true but reason is not the correct explanation of assertion
Pernicious anaemia is a deficiency disorder caused by low blood vitamin B12 levels. Viltamin B12 is required by the body to produce RBCs.
Cyanocobalamine is the man-made form of vitamin B12. It is used to treat pernicious anaemia.
Reasons gives the emphasis of assertion and not the explanation or justification.