**Core Concept**
The best test to diagnose fetal lung maturity in diabetic mothers involves assessing the surfactant levels in the amniotic fluid, which is crucial for preventing respiratory distress syndrome (RDS) in newborns.

**Why the Correct Answer is Right**
The correct answer is the L/S (Lecithin/Sphingomyelin) ratio test, which measures the ratio of lecithin to sphingomyelin in the amniotic fluid. This test is considered the gold standard for assessing fetal lung maturity. The presence of phosphatidylcholine (lecithin) in the amniotic fluid indicates the presence of surfactant, which reduces the surface tension in the lungs and prevents RDS. The L/S ratio is considered mature when it is ≥2.0. This test is particularly useful in diabetic mothers, as they are at a higher risk of RDS due to delayed fetal lung maturation.

**Why Each Wrong Option is Incorrect**
**Option A:** The Glucose challenge test is not relevant to assessing fetal lung maturity.
**Option B:** Non-stress test is a test of fetal well-being, not lung maturity.
**Option C:** Ultrasound estimation of fetal lung area is not a reliable method for assessing lung maturity.

**Clinical Pearl / High-Yield Fact**
It's essential to note that a mature L/S ratio does not guarantee the absence of RDS, and other factors such as prematurity and intrauterine growth restriction (IUGR) can still contribute to the risk of RDS.

**Correct Answer: C. L/S (Lecithin/Sphingomyelin) ratio test**