An old man, Om Prakash presented with anorexia, weakness, and paraesthesia. On further investigation, his hemoglobin came out to be 5.8 g% and the peripheral smear showed the presence of macrocytes and neutrophils having hypersegmented nuclei. His tendon reflexes also were sluggish. Endoscopy revealed atrophic gastritis. Deficiency of which of the following factors can lead to such a clinical situation?