You are answering a USMLE-style medical question. Question: A 28-year-old woman, gravida 1, para 0, at 20 weeks' gestation comes to the physician with her husband for a prenatal visit. Her pregnancy has been uncomplicated. They are planning to travel to Ethiopia next month to visit the husband's family. Medications include folic acid and an iron supplement. Vital signs are within the normal range. Abdominal examination shows a uterus that is consistent with a 20-week gestation. Which of the following drugs is most suitable for pre-exposure prophylaxis against malaria? Options: A) Doxycycline B) Mefloquine C) Chloroquine D) Proguanil Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.