Pernicious anaemia occurs in:
(A) Vit B1 deficiency (B) Vit B12 deficiency (C) Vit C deficiency (D) Vit D deficiency
(B) The most important disease associated with vitamin B12 deficiency is pernicious anemia. lt is characterized  by  low  hemoglobin  levels, decreased  number  of  erythrocytes  and neurological manifestations. One or more of the following causes are attributed to the occurrence of pernicious anemia.

Autoimmune destruction of gastric parietal cells that secrete intrinsic factor. In the absence of lF, vitamin B12 cannot be absorbed.
Hereditary malabsorption of vitamin B12
Partial or total gastrectomy-these individuals become intrinsic factor deficient.
Insufficient production of lF and/or gastric HCl, occasionally seen in older people.
Dietary deficiency of B12,  is seen among the  strict  vegetarians of  low  socioeconomic group in the developing countries (lndia, Srilanka etc.).

Key Concept: 
The most important disease associated with vitamin B12 deficiency is pernicious anemia.
Reference: Satyanarayana Biochemistry 3rd  ed page no 155