**Core Concept**
Fetal lung maturity is crucial for assessing the readiness of a fetus to breathe outside the womb. It involves evaluating the development of the lungs, specifically the surfactant production, which reduces surface tension and prevents lung collapse. Fetal lung maturity is typically assessed through various biochemical and ultrasonographic methods.

**Why the Correct Answer is Right**
The correct assessment methods for fetal lung maturity include:
- **Lecithin-sphingomyelin (L/S) ratio**: Measures the ratio of lecithin to sphingomyelin in amniotic fluid, indicating surfactant production. A ratio of 2 or higher suggests mature lungs.
- **Phosphatidylglycerol (PG) presence**: Detects the presence of phosphatidylglycerol, a surfactant component, in amniotic fluid. Its presence indicates mature lungs.
- **Fetal lung ultrasound**: Uses ultrasound to measure the thickness of the fetal lung and detect the presence of surfactant.
- **Kernicterus-free bilirubin**: Measures bilirubin levels in amniotic fluid, which can indicate fetal lung maturity.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not a valid method for assessing fetal lung maturity.
**Option B:** This option is not a recognized method for evaluating fetal lung development.
**Option C:** This option is not a correct assessment method for fetal lung maturity.

**Clinical Pearl / High-Yield Fact**
Fetal lung maturity is a critical factor in determining the timing of delivery. Premature infants with immature lungs are at high risk for respiratory distress syndrome (RDS).

**Correct Answer:** D.