You are answering a USMLE-style medical question. Question: A 23-year-old primigravida presents for a regular prenatal care visit at 16 weeks gestation. She complains of increased fatigability, but is otherwise well. She takes folic acid, iron, and vitamin D supplementation. Her vital signs are as follows: blood pressure, 110/70 mm Hg; heart rate, 86/min; respiratory rate, 13/min; and temperature, 36.6℃ (97.9℉). The physical examination is unremarkable. The complete blood count results are as below: Erythrocyte count 3.9 million/mm3 Hb 11.1 g/dL HCT 32% Reticulocyte count 0.2% MCV 90 fL Platelet count 210,000/mm3 Leukocyte count 8,100/mm3 Which of the following tests is required to investigate the cause of the patient’s laboratory findings? Options: A) Serum iron level B) Serum B12 level C) Transferrin D) No tests required Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.