The laboratory test to screen for gestational diabetes is:

A. Hemoglobin A1c: Hemoglobin A1c reflects average blood glucose over the past 2–3 months and is useful for diagnosing preexisting diabetes, but it is not the standard screening test for gestational diabetes. B. Urine glucose: Urine glucose testing is unreliable for diagnosing gestational diabetes because glucose may not appear in the urine until blood glucose levels are significantly elevated. C. Glucose Tolerance Test: The glucose tolerance test, typically a 1-hour screening followed by a 3-hour diagnostic test if indicated, is the standard method for detecting gestational diabetes during pregnancy. D. Random blood glucose level: Random glucose measurements provide a snapshot of glucose at a single moment and do not reliably screen for gestational diabetes, as blood sugar fluctuates throughout the day.