You are answering a USMLE-style medical question. Question: A 23-year-old woman, gravida 2, para 1, at 28 weeks' gestation comes to the physician for a routine prenatal examination. Her previous pregnancy was uncomplicated and she gave birth to a healthy boy. Her blood group is A, Rh-negative. The father's blood group is B, Rh-positive. Administration of which of the following is most appropriate at this time? Options: A) Anti-A IgG B) Anti-D IgG C) Anti-B IgG D) Anti-D IgM Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.