An old man having complaint of anorexia, weakness and paraesthesia. On investigation, his haemoglobin was 5.8g% and peripheral smear showed macrocytes and neutrophils having hypersegmented nuclei. His tendon reflexes were also sluggish. Endoscopy revealed atrophic gastritis. Deficiency of which of the following factor can lead to this situation
Folic acid Riboflavin VitaminB12 None