The main idea here is understanding what FBS stands for and what it measures. FBS means Fasting Blood Sugar, a test that checks the level of glucose (sugar) in the blood after you have fasted, typically for about eight hours. The term highlights two things: the fasting state, which ensures the glucose level isn’t affected by recent meals, and the analyte, which is blood sugar (glucose). This phrase is the best fit because it directly describes both what is being measured (blood sugar) and the condition under which it’s measured (fasting). The other phrases don’t fit as well: “fasting blood serum” would refer to the liquid portion of the blood itself rather than the glucose measurement; “fetal blood sugar” would be about a fetus; and “fasting glucose study” isn’t the conventional name used for this routine test, though it would convey the same idea.