A 26 year old female in her 4th month of pregnancy comes to her OB-GYN with a complaint of spotty vaginal bleeding for the past 3 days. Bleeding has been accompanied by mild nausea and vomiting over the past week. On pelvic exam, you note her uterus as being much larger than expected for her the estimated gestational age of the fetus. You perform an intravaginal ultrasound which shows a "snowstorm" appearance" and no fetal heartbeat can be detected. Which of the following is most likely to be elevated in the patient's serum?
b. Lactate dehydrogenase
c. Human placental lactogen
d. Human chorionic gonadotropin